Ok, so clearly I am doing something wrong then. I tested it with the english faction, I picked a hidden resource (from the descr_regions file) which the london province had, and another province of england didn't had (in this case tin). I put it in the row with the other hidden_resources in the export_descr_building file and picked out a english unit and have this as a condition. requires factions { england, } and hidden_resource tin. Also to make things more transparent I made the unit recruitable in every level of castle and city (So I don't have to make a barracks).
What happens is that every settlement is able to build the unit.