AACW features the concept of Chain of Command (more details later, as this is one of the major feature of the game). It simply means that the units in the American Civil War were organized more and more efficiently, within a hierarchy. With just an handful of rules, you will be able to manipulate armies, corps and divisions, each providing advantages. You can still detach units to act independently, but this will be less optimal.
Armies HQ are formed around a general. The better he is, the more powerful the bonuses given to attached corps will be.
Corps are attached to specific armies (Army of Northern Virginia, army of the Ohio, etc.). Each one is commanded by a corp commander, and receives bonuses from the parent Army.
Divisions and brigades are the fighting bricks of the corps.
Regiments, artillery batteries and cavalry squadrons are the smallest element of AACW. They are integrated into divisions and brigades, and can't be detached from them. You can still manipulate them indirectly by forming or splitting divisions into brigades.
Samples of the Union Corps available: