The plan is a bird's eye view of the set on stage after an imaginary line has been drawn 4' from the stage floor all around the set and stage. Then an imaginary crew comes along with a circular saw, cuts along the line and removes everything above the mark. None of this really happens, of course, but it is a good way to think of this drawing whether you are reading one or drawing one. So the plan is actually a "cut view" or horizontal section view of the set in place on stage.