"Plan Orientation" underlays:
The cut plane of the underlay is always taken at the level above the selected underlay level and looks down to the selected underlay level.
If you try to underlay the highest level then the underlay cut plane is set infinitely above the top level.
Here is a Plan View showing the Underlay beneath. As you can see the walls do not clearly show up.
I then change the Bottom Cut Plane to 8' this then changes the depth of the view range allowing me to see the remaining walls and doors.
For "Reflected Ceiling Plan" orientation the cut plane of the underlay is set to the selected underlay level and looks up to the level above the selected underlays level.