In descr_regions.txt there is a line missing from the format explanation at the top. It should read:
; region tag
; settlement tag
; default culture tag
; rebels tag
; map RGB value
; comma-seperated resource list
; triumph point value
; base farm level

The rebels tag (e.g. Brigantes, Trinovantes, etc) for each region links to descr_rebel_factions.txt. This file then defines what units can appear for a particular rebel army. It also defines non-region based rebels (Brigands, Pirates, and Gladiator Uprisings). Each entry has a 'description' value, which links to text\rebel_faction_descr.txt, where the localisation text for each rebel group is given.