Mod API Reference:SpecialUnit
SpecialUnit: Represents a special unit on the map, such as a commander or boss.
Note that players should generally not have more than 3 special units in their control at any point in time. This rule is not currently enforced by the game, but it may be enforced at some point in the future, so it's recommended that mods don't exceed that limit.
Also note that mods may not add special units to the game unless One Army Must Stand Guard is turned off.