Indicates if the subface instance is valid and can be used. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 16.0.0.0 (16.0.0.0) 
  Since:  2012 
  Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 16.0.0.0 (16.0.0.0) 
  Since:  2012