System families have a Function setting. That's true for Walls, Floor, Roofs and Ceilings. I've written about this in the past. Here's what they look like now.
I think the wording of a couple of the functions ought to be revisited, to something like this instead.
Finish 1  = Finish Exterior 
Finish 2  = Finish Interior 
If changed I suspect people would have less trouble deciding which function is appropriate. I run into wall types quite often that have these wrong or inconsistently applied. If you didn't realize this is how it's meant to work then check out the earlier post.