RTW vanilla has Rome, Sparta and Italy as hidden resources. My question is, how do you define a region as a hidden resource? And when the region is defined, the unit that has it as a pre-requisite can be recruited only when the whole region is owned or one city of the defined region is enough?

A good example of what I'm talking about is LOTR-TW. I remember when I played it that the units available changed according to the regions the cities were. But I'm not certain if that was because of some hidden resource or the "culture thing" (gondorian, ork, rohan, etc) each city had. Does anyone knows or suspects how it works?