If you have Second Sight or some other ability that lets you perceive across a Regio boundary, can you cast a spell on a target outside the Regio or in a different level of the Regio that you can sense? And if you can see them, is that Sight range or is Arcane Connection range required?
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
As Erik says, there is no single answer. Varying how it works by each regio seems best, so you can fit it to the story and so players won't be able to predict what will happen.
If/when a magus achieves a full understanding of how regios work, then that magus will know. Until then...don't try teleporting across the boundary! (Tends to be messy in my games.)