What Bengt says.
I understand that some building are Enclosed (?) in that you can't enter them if they have no doors, or no modelled interior. I think this is "you can't enter my apartment".
If it's Impassable, I think you can't move on it. You can fly over it. You can walk through it with Incorporeal (but not stop in the middle). But you can't stop on top of it.
I understand Impassable is the way you stop some terrain (like Sniper nests) from becoming too overpowered.
Back to the topic. Practiced Productions Trap Doors allows you to place a Shame Marker within 3". Not 3 or 3 or LOS and 3", but within 3". This means, I could place a marker atop a pice of Impassable terrain, which then makes it difficult to remove.