Judging by some of the replies it seems to me that real-world mobilisation needs to be explained:
In the US, we don't usually think of the term mobilization in this way. We think in terms of the National Guard and Reserves.
But they serve a similar purpose. You have a pool of trained manpower that can be called up for active duty, but they have civilian jobs.
A new mobilization system is really something to suggest for HOI5.
I think you could just do a whole DLC around it.
The way I would implement it would be something like this.
1) Conscription laws would come with two manpower pools (which would total to the current manpower they have). They would have the active duty portion and the reserve portion.
2) While at peace, active duty divisions could only use manpower from the active duty pool.
3) Reserve divisions can be created. They require manpower from the reserve pool. They use equipment like regular divisions, and they only appear on the map with X% of their strength.
4) When mobilizing, reserve divisions quickly get up to full strength and have more XP than they would normally from reinforcing to full strength (otherwise, they will have the worst XP due to replacing 90% of their manpower).
5) While at peace/not mobilized, you pay X% in consumer goods for each conscription law. The higher the law, the more CIC. Those reservists aren't available to work 100% of the time even if they have civilian jobs. This also helps balance the mechanic so that spamming huge reserve armies with massive conscription isn't free. The penalties to conscription we have now do not apply unless mobilized/at war.
6) When mobilizing, there is no more "reserve" manpower. It's all in the same pool.
7) Most free manpower in NFs will have to be abolished and should instead be replaced with a change in the ratio of active to reserve manpower from various conscription laws. This lets authoritarians "cheat" on manpower like they do now (generic focus tree) without actually getting free manpower (which has always bothered me).
8) Mobilization should normally only be done when at war, but I suppose some exceptions can be made. Regardless, mobilizing should add to WT like a DOW (without stacking with a DOW if you mobilize at peace) since mobilizing is usually seen as akin to a DOW anyway.
What's important here is that the manpower pools are only separate in the sense that you can't spend reserve manpower to create active duty divisions. If you want to
train that manpower, you have to form reserve divisions and exercise them at peace. We want to avoid trying to have "trained" manpower sitting around in pools, since that would really require HOI5.