Revit does not currently allow you to underlay a regular floor plan with a reflected ceiling plan. Why would I want to do that? Because I often want to show roof eaves or other architectural features that are above the cut plane of a plan (sometimes, WAY above). I want to parametrically show these features, but now the only way to do this is to manually trace these elements via model lines, or simply draw drafting lines in your view.
The underlay feature would be far more valuable if you could underlay plans and reflected ceiling plans interchangably. Ceilings and floor plans need to do more talking to each other.
JG