If I remember correctly there was a kind of "spill-over"-effect from an unit doing partisan suppression into adjascent regions... what again was the percentage of that "spilling over"?
Partisan Suppression
Well, by now I have enough data and can answere myself (and probably doing so within my DH-time for the 3rd time... I really tend to forget that number over and over again):
20% of the partisan suppression spills over in each directly adjascent region.
Example (DH full, 1914):
Infantry-3 (1907), suppression ability: 1
with military police brigade (Pol-1 from 1870): +3 = 4
doubled when ordered to Anti-Partisan Duty: x2 = 8
This unit suppresses 8% partisan activity in the region it is stationed and 0.2 x 8 = 1.6% (rounded up to 2%
(1)) in all directly adjascent regions.
All units around count for partisan suppression and their supression values are simply added together, may they be moving, fighting or directly ordered to suppress partisans. Partisan suppression is automatically done by units, the specific mission: Anti-Partisan Duty isn't necessary but very useful since it doubles the suppression value.
Placing and uprisings
The spill-over effect is why so many guides recommend to station your partisan suppressing garrisons in a check-board pattern. Garrisons are cheap and the best for partisan suppression but if you need heaps of them it is worthwhile to place them efficiently nevertheless. Unchecked partisans can lead to uprisings: Partisans take over the region, form militia units and instantly destroy any stationed planes there or take over that precious strategic naval bases you fought several month for. Thus, when placing your garrisons, priority should be given to regions you definetly not want to see taken over by partisans: airports with stationed planes, important naval bases, regions with lots of IC, resources. It is not efficient to bring down partisan activity to zero everywhere, so to cope with the occasional uprising, a few strategically placed mobile units are an useful addition to your main non-mobile partisan suppressing garrison force.
TC effects (Transport
Capacity)
The longer a war lasts, the more regions you conquere, the more you will see rather drastic effects this has on your available TC
and IC... both(2) tends to go down. This is caused by the accumulated effects of: Every conquered region takes a bit of your TC to manage, each region with partisan activity uses up a bit of your TC
and IC. Actually it is, while simplified, quite a good adaption of partisans striking at logistics, keeping busy troops etc.
Hover your mouse over TC or IC (symbols top right screen) to see the effects.
The partisan mapmode is very helpful. Be aware, though, that it doesn't show partisan activity which might occure in your allies' regions. To see partisan activity in your allies' regions you need to check the region itself and hover over the partisan symbol in the region window which shows max partisan activity and how much is surpressed.
Hint
When you start a war, when you expect it too last for a while with lots of conquered regions, especially non-national regions... kick your infantry from the building queues and replace them with plenty of garrisons. Add also a few long police queues. You'll need them. Garrisons are also fine for beach guarding and freeing up your true military units for the front. When I play Germany it is absolutely not uncommon that I run 6-9 garrison production lines paralelly (and mostly I wished for even more).
And you'll need your partisan suppressing garrisons longtime, partisan activity gets a bit lowered when the region status changes from occupied to annexed (=non-national) after peace but it won't vanish.
Example with a garrison
Garrison (Gar-2, 1907), suppression ability: 3
with police brigade (Pol-1, 1870): +3 = 6
doubled when ordered to Anti-Partisan Duty: x2 = 12
Cost: Roughly half in IC, daily supply and build time in comparison to infantry while giving considerably more suppression. Quite good for a unit you want to station once and then forget about it. Similar ratios apply for more modern units in the 1930ies.
Research
Technologically you get newer garrison-units as a side-effect from researching infantry: 1870, 1907, 1916, 1921, 1931, 1939...
Same with the police brigade but more seldom: 1870, 1921, 1944
(1) I don't know, though, wether the rounding up/down is only a display thing or also relevant for further calculations.
(2) correction: IC is NOT affected by partisan activity (but from dissent which is a bit misleadingly labeled Partisan Effect within the IC info break down)