Wal

The constructed space between the wall and the ceiling. Usually doesn't have a purpose, just for looks.

The two Ls in 'wall' represent how one wall represents how one wall acts as two walls, one for each room - a wall can only be a wall. The ceiling can act as either the roof or the base of the floor for the next level. The Wal, however, is neither, and can only be the Wal. It can only be present as separate Wals in one room (hence the singular L).

What is that weird piece between the wall and the ceiling? Is it a ceiling or a wall?

Oh, that's the Wal, it really doesn't have a purpose.