NTBF – New buildings for the towns in Tribes of the East 3.1.

Allow me. In the fourth installment, you could only choose one building out of the two available for recruiting creatures of each level (except for the first level). For example, in Necropolis (undead and demons), you could choose between ghosts and cerberuses, vampires and poisonous horrors, devils and bone dragons.
In the upcoming seventh installment, this mechanic is also present, but partially and only applies to higher-level creatures (champions).