I would wager that certain regions have certain mercenaries suited to that place historically and regionally.

For instance in vanilla if you were the Persians and you were invading the West you would use mercenary troops such as Thracians, Bastarnae, Merc Hoplites and Illyrians whenever you could for infantry, not your shitty hillmen and eastern infantry.

Considering the amount of thought the EB crew has put into this kind of stuff I would think they'd revamped the mercenary system to be much more dynamic then it was in vanilla and so foreign conquests will be all the more interesting for it.

In fact maybe thats what your 'occultus' is, mercenaries