The best way to do this is to have a table of users where the Key field is set to be the userid.
Then have any fields you like on that user field such as Region.
The magic is then you create a formula user field called [Current User] in any other table, with a formula of
User()
and make a relationships based on that field to the Users table. Then you can look up any user attributes and make a formula to control the access permissions with a custom Rule.
The beauty of this is that you get to create a single Role called "Allowed to see own Region Only, and it will apply to all those regular users who need to be limited to their "Own" Region.