Faction vs Regional Unit types?

I attempted to search this and got in over my head very quickly...

In short; I was playing Sicily and noticed that no matter WHERE i was i could raise Muslim archer militia ( even send them on Crusade) -
and so my question becomes that of national troops.

IS THERE ANY WAY to make -
myself i have aproblem with how militia is done so lets start with that as an example -

CAN I -
change militia troops types - say the muslim archers - to be REGIONAL rather tan a 'national' unit?

If i recall correctly; the Genoese Sailors could be raised by anyone that held genoa in MTW -
do the unit characteristics still work the same way or am i stuck with that as a 'national' troop type?
(the question of who will fight in defense - levy/militia - and what will work as standing army is a seperate but valid one; lets reserve it for elsewhere)

I just am a little unclear on which file or files controls which parameter.

What you have to do is to add a hidden_resource to export_descr_buildings, make your unit require that hidden_resource, and add the hidden_resource to the provinces in your world/maps/base/descr_regions.txt file where you want to recruit these units. Then delete map.rwm and let the game rebuild the file. This is a bit complicated because you have to change the vanilla map in some places, otherwise it can't rebuild properly (it may work if you don't delete map_heights.hgt, I don't remember it too well).