So now you’ve got a terrific building and you’ve got your site’s terrain modeled, and – consequently – you’ve got a grassy slope running through your building lobby.  This just won’t do!  That’s where the Revit building pad comes into play.  Even though you and I know the dirt should be retained at the exterior of a building’s wall, Revit doesn’t catch on to that.  You need to tell Revit where the boundary of your building is, so that it knows to cut a void out of the terrain.