Please Devs explain the supply system again for "dummies" like me..

HoI3 – The Supply System
The basic principle of the supply system in Hearts of Iron 3 is, at heart, quite simple. Every distinct, connected collection of provinces under the control of the same alliance has one province that is considered the Supply Source. This is the capital city in the home area and a port in all other areas.
When supplies are produced, they are proportionally injected directly into the core provinces that have IC in your home area. Supplies and fuel then propagate through the provinces by the following algorithm:
1) Units in provinces consume the amount of supplies and fuel they need from the amount available in their province. (If there is not enough, the units start going ”out-of-supply”.)
2) Starting with the provinces furthest from the Supply Source, all provinces try to draw the total amount of supplies and fuel required from surrounding provinces, first asking the province(s) closest to the Supply Source. The amount required is the sum of supplies and fuel needed by units in the province and the amount asked for by neighboring provinces.
3) Surplus supplies and fuel in provinces are drawn back towards the Supply Source.
So far so good, but there are two main factors complicating the system. First, the level of Infrastructure in a province puts a cap on the amount of supplies and fuel that can pass through each day. This can make it difficult to supply large armies in areas with poor Infrastructure, or past
Infrastructure bottlenecks. Secondly, there is a ”tax” on supplies and fuel each time they are drawn to a neighboring province. This means that the further away from the Supply Source your units are, the more expensive they are to supply, and the greater strain they will put on your Infrastructure.

Supplies from Allies
If your units (this also applies to expeditionary forces you have sent) are in an area that has an allied province as Supply Source, they are effectively drawing supplies from that country, which could put a huge strain on their industry. What happens in this situation is that an automatic supply trade is created between you and your ally, where you are giving them the amount on supplies and fuel that your troops are drawing from them. Unfortunately, this trade is invisible and does not require a supply convoy, meaning it is safe from enemy predation. Hopefully, a better solution will be found down the road.

Supply Convoys
Supply convoys require an amount of transports proportional to the length of the route in order to operate at max capacity. Max capacity is determined by the port size in the end province only.
Provided it has enough transports, the convoy will operate at needed capacity, pumping in as much supplies and fuel as needed in the area, but limited by what the receiving port can handle.
If the troops in the area need more supplies than the Supply Source port can convoy in, additional convoys can be set up to other ports in the area. However, it is important to remember that the area still only has one Supply Source. This means that supplies from secondary ports might curve back
towards the Supply Source on their path to the units that actually need them. This is quirky, but should not pose much of a problem.

Why not trace direct Paths?
People often ask for a supply system where each unit traces its own path to the nearest sufficient supply depot, and then pull supplies from it. This would be more accurate and would avoid the quirks of the HoI3 system. However, it would also be tremendously costly in terms of CPU use, and
would make the game completely unplayable. (In computer science, the problem is known as the ”bottleneck travelling salesman problem”.)

Supply and the Arcade Mode
In Semper Fi, it is possible to set up convoys between different ports in the same map area that contains your capital province, provided they are more than 10 provinces distant from the capital. For example, the Soviet Union can now set up a convoy between Leningrad and Vladivostok. Since supplies are ”sucked” towards the units that need them, this can be useful if a province is very distant from the Capital (although the path the supplies travel from the receiving port of the convoy to the units that need them might take a short bend back towards the capital first, due to how the system works.)

If you would prefer to avoid the headaches of logistics altogether, there is a new option in Semper Fi called ”Arcade Mode”. Before you start a new
game, you can choose between Normal or Arcade Game Mode. In Arcade Mode, units draw fuel and supplies directly from the national stockpile, without
penalties, regardless of their current location. Fuel and Supplies are not infused into the map provinces, the Logistics Map Mode is disabled, and supply convoys can no longer be set up (because there is no need for them.)

Supplies & Fuel
The information on the Supply Mapmode has been improved. Before, there was a lack of clarity as to what each color represented. See the graphic to the right for more detail. The system of getting supplies to your units has been revamped in a number of ways. The movement of supplies across borders has been improved, so that you won’t start running out of supplies as you move across allied land. Overall, it seems a lot harder to cut units off from supply – both to pinch supply off by narrowing the corridor, and also by surrounding units, they seem to retain their supply status longer.

New Supply Lines
Tere’s no longer just one original distribution point for supplies and fuel (which used to be your capital). Now, any core IC (i.e. industrial centers within your home area) province which is connected in any way with your capital can be a source of supplies or fuel, so if part of your coun-
try begins to get cut off, you still have a portion of your supply that can reach that area. If these core provinces get cut off from the capital
by enemy action, so that you cannot trace a line of controlled provinces to it, then it will still be cut off from supplies. On a practical level this may not be the disaster it used to be, because units in v1.3 still seem to be capable of independent action for a longer period of time without
having access to renewed supply. Lastly, units which run out of fuel will not get completely “stuck” in v1.3 as they did in earlier versions.
Tose units which should get stuck because they’re completely cut off, will still be stuck, but those which in previous versions would seemingly have access to fuel but didn’t get it should not recur in v1.3.

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The Supply Mapmode: In the revamped mapmode, gray is the default color for enemy or neutral territory (places where you are unaware of the supply status because they’re not yours). Green indicates there is an active supply line where supplies are getting through and meeting demand. Brown is an area where there is demand which is not quite met. Yellow is a more serious shortage, and red indicates units which are totally out of supply. You will no longer see red in provinces where there’s no demand to be met. A blue color indicates there is a surplus of supplies.

Paradrop Supply
For those of you who might have experienced out-of-supply penalties when paradropping, this will now not happen until they’ve used up their inherent supply stockpiles.
Another situation some may have run into is combining two paratroop divisions in a drop, you might have found them “attacking from different directions” and therefore adding to the combat frontage. Tis has been fixed, and they will fight together as if they share a landing spot.

[20.0] SUPPLY & LOGISTICS
Summary
Veteran Hearts of Iron players are familiar with the concept of Transport Capacity, which has been eliminated in HOI 3 and replaced with a much more
realistic system, as follows…

Also note that with Strategic Redeployment, your units no longer use the “beam me up” system, where they disappear and later reappear in another location, but instead in HOI 3 units are hurried along fast railway lines where they are visible and vulnerable to enemy air attack.

[20.1] INFRASTRUCTURE
General Rule;
Infrastructure is the system of roads and railways which make it easier to get from place to place without getting your feet all muddy. Infrastructure greatly affects important things such as how quickly units can move through the province, and how efficiently Supplies can transit through en route to your units.
Commentary;
Infrastructure is a measure of how “built up” the territory is and, more to the point, how easy it is to transport military forces or supplies through the province. A simple “line-of-sight” to your supply source does not guarantee a unit will be in supply – there has to be sufficient “throughput” (which is supported by infrastructure) to allow enough supplies to get through for all the units reliant upon those roads or railways.
Yes, you can stack 100 divisions in one province, but if you do they’ll start losing strength because you can only supply a few of them. Units which
can’t get supplies cannot fight attrition with reinforcements, nor may they recover organisation, if they’ve recently been in battle.
Units in countries which have low infrastructure will constantly be fighting to get enough supplies. On the long-term, the way to improve this is by building higher infrastructure, or building higher level naval bases if the theatre is overseas. On the short term, more highly skilled commanders at the corps level, and higher throughput technology may help. Sometimes, just letting units in a region rest may allow them to regain their supply, because units which are sitting still use supplies at a slower rate than those in combat. Another option would be to open a new source of supply nearby, which uses different the throughput from new provinces to transport supplies. This might be done by capturing a new naval base, or even constructing one along a friendly shore. Long term combat strategy requires strong infrastructure channels through your country which can be used to get your armies where they need to go. Invasions of another country are made easier by strong infrastructure there, as well. On the other hand, you always run the risk of allowing your enemies to use your infrastructure against you if you are on the defensive. You should build infrastructure
in areas you feel are secure, but where you expect to need to support large numbers of divisions. Once captured enemy territory becomes secure, it would be good to improve the infrastructure there, too.
The rate at which your units repair is also dependent partly upon the infrastructure in the province where it is. However, unlike previous games, the rate of resource production in a province can only be improved through technology.
Be especially careful while using “forward airbases,” where the base is very close to the front lines, because these bases will draw upon the same
supply throughput as the units at the front. You may find your air units and your land units are interfering with each others’ supply.
Supply tax can be a serious drain on your Supply stockpile, so as your empire grows or as your armies move into enemy territory, your supply needs will gradually increase. It will get worse if rain or melting snow is causing mud along your routes.
Supply is easier through provinces you own, (i.e. not in occupied territory).
If you’re having systemic supply problems, like armies in Russian or Chinese territory often do, attacking only makes the problem worse, because it
increases the draw on supplies. Resolve your supply issues first, and then attack. If there’s no way to effectively solve the issue, then attack sparingly or invest your attack strength into trying to open better supply routes.
If you’re wandering around behind enemy lines, it’s possible to capture enemy supplies and make use of them yourself. You may collect those supplies
moving through the province at that moment, or you may even be able to capture an enemy depot.
It’s sometimes useful to take just a few days to prepare an army for a new offensive, because if you’ve just moved forward it’s going to take a
couple of days for the supply lines to adjust and fully prepare you to move forward unless you are willing to do so without a fully established supply line. Your units may also need to refill their 30-day reserves, and this period does well to restore any strength or organisation they may have fallen behind on, too.

Cases;
[20.11] A province’s Infrastructure level is displayed in its Province Interface, which can be seen by clicking on the province.

[20.12] There is also a useful Infrastructure Mapmode that provides a good overview.

[20.13] Infrastructure represents how “built up” a province is, or perhaps “how civilized.” Improving things like roads, railways, and communication
networks in a province will increase that province’s Infrastructure level.

[20.14] Being the site of Combat, bombings, or other hostile attacks can reduce a province’s Infrastructure level.

[20.15] New Infrastructure may be constructed in Provinces (Hexes) by clicking on the Infrastructure icon in the province screen, which will enter the Infrastructure construction into the Production Queue. It will take one year to produce and cost 1 IC. Once constructed, it will gradually come into being over the course of several days as a new point of Infrastructure.

[20.16] Damage to a province’s Infrastructure will temporarily lower the effective ability of that province to meet its potential Supply Throughput.

[20.2] THROUGHPUT
General Rule
Throughput is the volume of Supplies you can fit through a province per day (which is enabled or limited by the Infrastructure level). Imagine two-ton trucks lined up bumper-to-bumper on a narrow, muddy road.

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“However will we explain this to the Führer?!” The cut-off province of Orynyn shows a need of 3.56 Supplies, but a Throughput of zero, which is bad for the surrounded Unit (it will draw from its 30-day reserve at first).

[20.21] Your Supply Throughput is limited by the province with the lowest Infrastructure along the Supply path, because it becomes a bottleneck. Your
Supply sergeants will try to route through other paths if one becomes too crowded, but on busy fronts, those other routes may experience traffic
jams too.

[20.22] Some Technologies may allow you to increase your Throughput of Supplies per Infrastructure point.

[20.23] Technologies may also reduce each unit’s Supply consumption.

[20.3] SUPPLY & SUPPLY LINES
General Rule
Every province, and every Unit within that province, draws supply from a central province in your rear area. Supplies for your armies must be able to move from their centralized Depot to your units without being intercepted. Supplies move along the Supply Line at the rate of one province per day, until they catch up to the unit they are supposed to Supply.

[20.31] Moving Units & Supply Lag
When a unit moves, its new location filters back through the lines, and new orders for Supply are directed to the unit’s new location. This filtering
process creates a lag, which may delay the Supply of the unit. It may take a day or two for the Supply Line to fully adjust and begin delivering supplies to the unit’s new location.
If new units which require supplies move into a province, you may see the Mapmode colours turn from green (Supplied) to brown (partially Supplied),
indicating that the rate of Supply cannot keep up with the increased demand from the additional units.

[20.32] Supply Routes
The Supply route is flexible. If the logistics management system cannot draw sufficient Supply through one province, it will search for other routes
to deliver the necessary amount.
Supply can’t go through lakes or other large bodies of water (except for straits, unless they’re blocked by enemy naval units), so large lakes can be useful terrain features if you’re trying to isolate or constrict supply to a group of enemy units. Examine the infrastructure map to see which provinces are obvious routes for supply, and use your divisions and airpower to try to cut off those routes.

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Pretty sure it follows the shortest route from source to destination. Doesn’t care about infra levels. I think you can determine the route by clicking on a unit in the source location and issuing it a movement order to the destination.
You can test verify it by looking at the supply map - you should see a green route for the most part from the source to your dest. - at least parts of the route should be green
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[20.33] 30 day Supply Reserve
Units have a 30-day reserve of Supplies, which may become exhausted if the unit is constantly running ahead of its supplies.

[20.34] Overseas Supply
Overseas Supply is limited by the size of the Seaport taking in the Supplies. Sometimes, though, because of the sheer length of overland Supply lines, it may still be more efficient to direct Supply by sea.

[20.35] Supply Tax
Since the process of transporting Supplies uses Supplies (Fuel, etc.), there is a small Supply cost per province, which can add up over distance. If any part of the Supply Line route passes through Muddy Ground, that portion of the Line is impacted by an extra cost of transportation (“supply tax”).
There’s a higher supply tax for provinces you control but do not own (i.e. occupied land).

[20.36] Logistics Technology
Technology Research can improve your ability to move Supplies. It can also reduce the Supply consumption rates of your units, though good Leader
Skills or Traits can do this as well.

[20.4] OUT OF SUPPLY
Units that are out of Supply will not be able to recover Organisation, will not Reinforce, and cannot Upgrade. What units are or are not in Supply is
recalculated each day.
Units which find themselves temporarily out of supply may suffer from partial Unsupply, and will feel the effects of that inefficiency. These units will draw upon their 30-day reserve before they actually go out of Supply (i.e. Before they start feeling the effects).
Extended periods of time outside of Supply will seriously degrade the fighting Strength of the unit.

[20.41] Supply Mapmode
It is highly recommended that you examine the map in Logistics Mapmode to get an idea of how your Supply situation is being handled. The use of the
Logistics Mapmode is explained in Case [4.77]. Overseas Trade Routes are marked with red and blue lines, extending the length of the Route.

[20.5] OIL & FUEL
HOI 3 separates the traditional Resource of Oil so that there is Crude Oil on the one hand and Refined Fuel on the other.
Crude Oil is a Resource produced at certain locations, which must then be transported back to the home country, where it will be Refined at a rate
that depends on your actual IC, modified by Technology.
Refined Fuel will then become available to your units at home, and can then be transported to your units via Convoys or Supply Lines.
You can Trade for either Oil or Fuel on the world market, which is the preferred method of meeting your Fuel needs; it’s cheaper than converting from
Energy.

Every country has a limited ability to Convert Energy Resources to Oil, which basically takes the Energy and turns it to Oil using a wastefully
inefficient ratio. It is preferred that you meet your Fuel needs by refining Oil.

[20.6] CONVOYS
Summary
If your country has territory in a location geographically separated from your capital, you will need to maintain some sort of Supply connection. If
a connection is not maintained with the distant territory, you may quickly lose it during wartime.
Presumably, you will also have some military forces assigned there, which will need Supplies to operate.
As noted in B3.0, it is the buyer of a resource on the world market who must transport those goods on his own transport ships if the goods must be transported overseas.
You can only attach escorts to convoys while you’re at war. Otherwise they sit in a pool, waiting.

[20.61] Assigning Covoys
To send Supplies overseas, you will need to build and assign Convoys. Conveys are made of collections of cargo ships and tankers that carry
Supplies and Fuel in groups, making them easier to protect and more secure from enemy attack.
Resource Trades also require Convoys. The country making the offer is also required to provide the ships to carry the Trade. If the Convoy cannot make it to the purchasing home country, those items Traded for are lost.
Convoys are managed through the Production Interface. At the bottom-right is a list of your current Trade Routes, describing the Resources being traded but not the Convoys themselves. Below that is a detailed list of all of your Convoys, both those for Resource transport and Supply transport. The list shows origin, destination, cargo carried, and the number of assigned Convoy points and Escort points. The number of Reserve Transports and
Escorts are listed above the display.
In order to set up a Convoy, click on the “Create Convoy” button just underneath the Available Escort figure. A display will come up, asking what the
Convoy’s Origin will be with a list of origination Seaports. Once you select an Origin, you must select its Destination in the same way, by picking from a list of overseas Seaports Controlled by you.

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Convoys can be created automatically, if you select that option, so long as you have available Convoy Ships. Escorts will be added, too, if they are
available.

A formula is used to determine where along a Convoy Route the Convoy is actually located. If there are enemy Submarines or surface ships in the
Seazone, there is a chance they may be able to attack the Convoy. If a Convoy is damaged, the Resources or Supplies being carried will be lost in proportion to the number of Freighters sunk.
If a Seazone starts to get hit with attacks by Submarines or a Convoy Raider, the Convoy management system will automatically adjust the
Convoy Route to avoid the dangerous area. This is an ongoing process that should make Convoy Raiding a guessing game for the enemy.
Aside from the Convoy Escorts assigned through the normal Convoy system, Ships from your Naval Bases may also be assigned to Patrol and guard
specific Convoy Routes. These Ships will travel along the assigned Route and try to engage any Submarines, but this will be more difficult than
locating surface Ships. Any Class of Ship may be assigned to this duty, but Destroyer Flotillas are the most effective at finding Submarines. See Case [33.11] for more information on this Naval Mission assignment.

[20.62] Naval Bases
The size of the Naval Base determines how large the stockpile can be, as well as how fast it can replenish its stockpile.
It also determines how quickly Ships which Base there can regain Organisation and Repair their Strength.

[20.63] Convoy Escort Example (See Examples)

[20.64] Supply/Fuel Depots
A Supply Convoy can leave a Stockpile of Supplies at a Naval Base, from which all nearby Provinces (Hexes) will draw their Supplies. This will be the
central distribution point, and all nearby units will have to maintain some kind of contact with this Depot in order to remain in Supply. There are no overseas Resource stockpiles – you must ship all Resources home, or they will be lost.

[20.65] Escorts
You will also want to build and assign Escorts to protect your Convoys from Raiding and Submarines.
Otherwise, your Convoys may be sunk faster than you can build replacements.
Escort points are produced the same way as Convoy points, and Escort points are produced and assigned the same way as Convoy points.

[20.66] Logistics Interdiction & Convoy Raiding
Land units may interfere with enemy Supply Lines by physically occupying Provinces (Hexes) through which the Supply Lines travel. It is possible to
capture or destroy Supplies when overrunning enemy Supply Lines. A portion of captured Supplies may be used for your own units – this may be just
what you need if you’ve advanced ahead of your own Supply Lines!
Logistical Airstrikes can create additional hardship for the Supply of outlying units. This is done by Bombing the Infrastructure of Provinces(Hexes)
along the path of Supply. These Logistical Strikes temporarily destroy Infrastructure and permanently destroy Supplies, meaning the enemy may be out of Supply for a day.
Convoys, of course, can be attacked at sea by enemy naval forces such as Submarines or Convoy Raiders.
Aircraft can also attack Convoy Routes at sea.
Sinking Convoys will interrupt the provision of Supplies by sea, and may place units out of Supply.

[20.7] FLEET SUPPLY
[20.71] A Fleet is always connected to a home Naval Base, and draws its Supply from that Base, even while it’s not there.

[20.72] So long as the Base has sufficient Supply for the Fleet, and the Fleet has not surpassed its maximum range before returning to Base, it will
remain In Supply.

[20.73] If it is unable to reach a Naval Base before surpassing its maximum range, its speed will slow considerably.

[20.8] AIRDROP OF SUPPLY

[20.81] There is an Air Mission for Transport Planes to deliver a limited number of Supplies by air to distant or cut-off units.

[20.82] The Supplies are drawn from what’s available at the Airbase from where the flight originates.

[20.9] STRATEGIC REDEPLOYMENT
General Rule
Strategic Redeployment is a method of quickly moving units by train from one area to the next.
Commentary;
Land units using strategic redeployment will move at what’s essentially a speed of 20, modified by the infrastructure level. They will lose 1 point of organisation every day. Supply costs will be double.
Theoretically, there is an infrastructure-based cap on strategic redeployment, because these units will still need supply (in fact, they will need more of it) to travel along their route. However, they will not need fuel, which may be a useful point.
Consider using strategic redeployment to move your units if you’re short on fuel. Not only is it faster, but it also costs you supplies, rather than fuel. You can SR most places, including into combat, though that’s risky because you also lose organisation. You can come to just within range, though, and then move forward.
SR reduces the organization of your units, so they will not be as prepared to fight. Because of this it’s a bad idea to use strategic redeployment to carry your units straight into combat. But there’s no rule against it, and at times it may seem a useful tactic to get your units into combat more quickly.

Cases;
[20.91] They may move ahead of their Supply line and will carry Supplies with them, which should hold them over until Supply can be re-established.

[20.92] Strategically Redeploying units will move them at a speed of 20 KPH per point of Infrastructure level, so actual speed will vary
according to the Infrastructure levels along the route.

[20.93] These units will lose 1 Organisation per day, and will use twice as much in Supplies.

[20.94] Units that use Fuel, such as tanks, will not consume Fuel while being Strategically Redeployed.

[20.95] To strategically move a unit press Ctrl and select the unit.

[20.96] Strategic Redeployment does not cost fuel – it costs additional supplies. It also reduces organisation, and may also cause the unit to have an attack delay.

[20.97] Units move at a speed of 20, which is applied differently depending on the infrastructure of the province (just as with any other unit’s speed).

[20.98] Can not SR through another country. One must SR to your border, then move into the country's province, then SR in that country to to the border, then move out of the country into the next province.