Haven't played with this with this is a while so I could be wrong but I believe it's related to this... except that it's what allows the RIF to map a Revit Family to a specific ITM where as this like explains it when the conversion is done in Revit.

https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/CloudHelp/cloudhelp/2018/EN...