Define a ReferencePoint relative to a planar reference. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
Syntax
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Remarks
 A ReferencePoint on a plane has its X and Y basis vectors parallel to the plane, but is free to rotate around its Z basis vector. It can lie on the plane or have an offset.