Never mind, I received an answer.
"Q selects snap point, T changes "type". Wall can be a wall, doorway, window, fence, etc. Roof can be roof, stairs, roof door, ramp, etc."
So a wall can be a fence, but a fence foundation can't be. A roof can be stairs, but wooden stairs can't be.
Yeh, that makes a lot of sense.
Who proof reads & organizes this stuff?
LoL