An unset BoundingBoxUV has its lower-left corner to the right of (and above) its upper-right corner (this is an arbitrary internal representation). It means that it is empty and has never been initialized. If someone using the API creates a Plane via one of the Plane.Create() methods, for example, the Plane will not have its envelope set, so Surface.GetBoundingBoxUV() would return an empty BoundingBoxUV.