Id of the work plane which may determine the orientation of the load. 
   Namespace:   Autodesk.Revit.DB.Structure  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2015 
Syntax
Remarks
 The load might be oriented to be perpendicular to this work plane. Determine how load is oriented by checking  OrientTo  .