The best material id determined for the family instance. 
   Namespace:   Autodesk.Revit.DB.IFC  
  Assembly:   RevitAPIIFC  (in RevitAPIIFC.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2012 
  Namespace:   Autodesk.Revit.DB.IFC  
  Assembly:   RevitAPIIFC  (in RevitAPIIFC.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2012