The id of the base level of stairs on which the NumberSystem is placed. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 18.0.0.0 (18.2.0.0) 
  Since:  2018 
Syntax
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Remarks
 In multistory stairs, a stairs element could be a stair group or individual stair, this level could be the base level of group member or stairs element.