well, according to the games, in each generation you are assigned to a region where you fight the evil and then you are to fight the elite four of that region to become the "Pokemon champion" so, for example, if you completed x or y your champion region would be Kalos since you would have only defeated the Kalos area elite four. In this roleplay, every region has an elite four, creating more than one "Pokemon champion" so wherever they completed in getting all their badges and becoming the Pokemon champion would be their "mastery region"
Hope this cleared that up, and I also gave you a little backstory in the. reasoning why I made it that way. Sorry for the history lesson dear :p