Decided to test this myself, here's the results with some screenshots:
On this flat wall, on this side it shows the corner in the indicator as yellow, which provides 100% cover as the wall does:
As far as I can tell, that tile is the one the wall is actually placed in.
The other side shows the corner as grey, and in a mirrored scenario the corner provides no cover whatsoever:
The empty tiles beside them have their colors in the indicator reversed for that corner:
This behavior seems to be mostly consistent. There are however some oddities which I can't tell why they happen, like this wall corner that can't be leaned from:
And this case with two different colors on each side:
Overall, this doesn't seem to make much sense.
