Returns the load operation GUID. 
   Namespace:   Autodesk.Revit.DB  
  Assembly:   RevitAPI  (in RevitAPI.dll) Version: 2015.0.0.0 (2015.0.0.0) 
  Since:  2015 
Syntax
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Return Value
The load operation GUID.Remarks
 This Id uniquely identifies the load request. IExternalResourceServers can use it as a key to store and retrieve information (such as errors) that is relevant to a specific load operation.