It's common to keep a reference number on objects to relate them to their original database. This should be in a "ref:AB:XYZ" tag, with AB the country and XYZ the name of the external database. Here maybe "ref:FR:onf" instead of iidtn_frt.

This one tag is sufficient to relate every object between OSM and the external database and to look up further information there.

I have no idea what the other three keys might be - they should have a documentation in the wiki, preferably together with the documentation of the import from this database.
