to the best of my knowledge there is no answer to this in the published material.
I think that the best answer is: it depends on the regio
It has come up before
https://forum.atlas-games.com/t/regio-boundaries/955/2
https://forum.atlas-games.com/t/regio-borders/4654/9
https://forum.atlas-games.com/t/regiones-and-arcane-connections/801/1