At the start of each game, excepting certain scenarios, the player can click the circular button at one corner of the minimap to bring up a screen like the one to the right. They then have to pick one of three doctrines to use for the duration of the game--this cannot be changed midgame, so it is an important decision.
As units earn experience, they not only make themselves more skilled, but also contribute that XP towards Command Points. Each doctrine tree consists of two paths, and each path contains three abilities, all of which cost a number of Command Points to unlock. These can be passive, giving boosts to unit stats or unlocking new unit powers (for instance, the Panzer Elite cannot build Flak 88 emplacements without unlocking them in the Luftwaffe tree), or they can be active powers known as Offmap Abilities.
Offmap Abilities appear next to the Command Point button when unlocked. They come in two broad categories: ones that call in special units, and ones that activate special powers. For instance, the Americans can drop Paratroopers, and call in strikes from long-range Howitzers. You can review a doctrine's unlocks before choosing it, so take the time to look them over.
Most Offmap Abilities require resources to use--often Munitions for airstrikes and artillery barrages, but also Manpower and Fuel for various units. Special units can also not be summoned without appropriate population cap, so pay attention!