The banner carrier is defined in gondorinfantrybanner.ini.
In the menhordes.ini. Gondorfighterhorde, you'll find "Behavior = HordeContain ModuleTag_HordeContain".
Beneath that you'll find:
//Banner Carrier info
BannerCarriersAllowed = GondorInfantryBanner
Followed by the unit's position relative to the horde.
At level 2 the horde receives the basic training upgrade which allows the banner carrier to spawn.
thanks, I knew that, looking for more deep detailed info