By definition a transom is "a window set above the transom of a door or larger window; a fanlight" . I believe that if you take a Marvin window for example and add a transom, Marvin will add the transom above the chosen window thus leaving the window per the IFC dimensions. If the transom is taken out of the window the IFC dimensions no longer hold. The VW interpretation messes with the window dimensions in elevations, schedules, etc unless you manually change the window height and elevation to acommadate the transom.