We been importing some customers old ERP data via JSON RPC and generating external ids to database. Like the importer does, except we add own module to ir.model.data  

Example data is:

{"name": "EXTERNAL_ID",                "model": "product.template",                "module": "erp_importer",                "noupdate": False,                "res_id": "20"}

But now we get erros when we are trying to update our erp_importer module, because noupdate is False. It can be changed to True and after that it works well.

Now we are wondering what is the purpose of noupdate?