AircraftTypes, BuildingTypes, InfantryTypes and VehicleTypes
Red Alert, Counterstrike and Aftermath
Specifies a list of sides which should be able to construct this object. Recognized values for sides are allied, allies and soviet.
Tiberian Sun and Firestorm
Specifies a list of houses which should be able to construct this object, or gain this object as a multiplayer starting unit (if it also has AllowedToStartInMultiplayer=yes) or from a bonus crate.
Red Alert 2 and Yuri's Revenge
Specifies a list of countries which should be able to construct this object, or gain this object as a multiplayer starting unit (if it also has AllowedToStartInMultiplayer=yes) or from a bonus crate.
If a player owns a Factory=(UnitType, InfantryType, AircraftType or BuildingType) building with at least 1 Owner= country that both factory and unit share, then given you fulfill all the other Prerequisites you can build the unit in question.
If this list includes one country from Side A, it should also include the other countries of Side A. More granular permissions should be applied via Prerequisite, RequiredHouses and ForbiddenHouses.
BuildingTypes generally include all multiplay-avaliable countries in this list. This is not completely necessary, but it can help new modders avoid the New Construction Options Bug in some cases if they keep this format.
If a country has the proper Prerequisites but does not have a listing in a unit's Owner= list this will result in the New Construction Options Bug.