Manpower

A nations' manpower is its men eligible for military service that are used to create and reinforce armies.

Maximum Manpower
Maximum manpower is a sum of your provinces' manpower pools, where that pool is derived from class manpower of each classes in each province. Manpower tax determines how much of each class are extracted as manpower.

Manpower Recovery
State manpower recovery can be seen as actually flexing your administrative muscle to keep track of people who should be raised as levies by right. Class manpower, on the other hand, can be seen as demographic recovery where new people are born to fill up the place of the deceased. When class manpower is below its maximum amount, it will automatically regenerate.