Every modder define this thing.
For example, in Sparta, only "Spartiatai Hoplitai" will be recruited on the city while "Perioikoi" or "Heilotai" can be hire in the province...
The same like "Sacred Band" in Carthage, "Leukaspidai", "Hetairoi", "Argyraspidai", etc.
But this is only my opinión.