Typically for IFC export, this represents the lower and upper elevations for split wall and column geometry. 
   Namespace:   Autodesk.Revit.DB.IFC  
  Assembly:   RevitAPIIFC  (in RevitAPIIFC.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2013 
Syntax
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Remarks
 It contains two double values representing the starting and ending values as a range.