thanks jeremy for your response,
I try to explain what i mean in the attached picture.
Rebar in revit is equivalent to IfcGroup in Ifc File (Of course if all members of the group are bars)
single instance of Rebar is equivalent to IfcReinforcingBar in Ifc file
also we know that the source of the ifc file is revit.
so, some how, the guid of the instances should be some where in Revit API, and, if it doesn't so, it should be generated by Ifc exporter.
I have no idea how Revit generates these GUIDs.
