Every kingdom belongs to a "Region" and can use the units available to the region.
Some kingdoms also have their own special units.
These units are determined by the country you choose at the start of the game, and can be hired in any town you conquer during the game.

Similarly, all towns belong to a "Realm" and can use the units available to the realm.
Some towns also have their own special units.

All unit status list in UltimateHD edition.