The text shown below the segment's value. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 16.0.0.0 (16.0.0.0) 
Syntax
Remarks
 This property is for use with single-segment dimensions only. For multi-segment dimensions use the corresponding properties of the DimensionSegment class.