This method will return the fabrication id. This represents the link between the Revit and the Steel Core element. 
   Namespace:   Autodesk.Revit.DB.Steel  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 19.0.0.0 (19.0.0.405) 
  Since:  2019 
Syntax
Remarks
 This id should not be confused with the guid returned by ExportUtils.GetExportId().