If you have ever wondered why some walls don't show up in underlays or show up inconsistently from floor to floor, or why roofs and view ranges affect their behavior, then read on.
First rule of underlays: Underlay view extent properties (cut, range, depth) are not user modifiable and not coupled to any other view's properties.
For "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.
"Reflected Ceiling Plan" orientation:
The cut plane of an underlay is set to the selected underlay level and looks up to the level above the selected underlays level.
It's also been mentioned "If objects are visible in the view range of your current view, AND in the view range of your Underlay, they are automatically hidden in your underlay, so there isnt a double instance."
I can find no evidence of this. Instead, the object's line style switches from an override state to normal object style when the object is within the view range.
I hope some of you find this useful.