Unfortunately they aren't sorted, I recently made some major modifications to Mundus map for the Roma Surrectum mod, and removing the regions I didn't want was a pain because they were all over the place.
An easy way to find the region(s) you're after is to look up the colour code in map_regions.tga and then search for it in descr_regions.txt. If a regions has the colour R:100 B:101 G:102 search descr_regions for 100 101 102 and you'll find the region.
EDIT: In fact what I ended up doing was writing down the colour code for every regions I wanted to remove from map_regions.tga and then one by one removing them from descr_regions.txt and other necessary files.
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