I'm not sure what you are asking when you talk about a RIN. The GEDCOM standard defines the data that is transfer with this tag as:
"A unique record identification number assigned to the record by the source system. This number is intended to serve as a more sure means of identification of a record for reconciling differences in data between two interfacing systems."
The tag looks like this "1 RIN (AUTOMATED_RECORD_ID)"
FTM drops this tag without tell us.
Remember that GEDCOM is used to transfer information between systems and as such would require that both systems maintain information about where information came from and where it was sent. In this case the receiving system (FTM) would need to look and see if the unique ID (the RIN) that is being transmitted now has the same RIN as information that was transmitted at some time in the past and by that match know that this new transmittal is an update to a previous transmittal.
I can't say for sure that this works or does not work at LDS, my guess is that it does and they use it for their purposes. However I do not think that this tag is reliable for use by desktop systems like FTM where inport and export of GEDCOM trees are not accumlulative. If FTM and A.COM used GEDCOM to transmitt information the RIN tag may be of some use, but I don't think GEDCOM is used.
RIN has mistakenly been thought of being the same as the XREF number used internally by a decktop software application to link data elements together, aka a "foreign key" or "pointer". The XREF is mistakenly labled RIN in some systems. An XREF can be anything including all letters. A true RIN is unique to that record type and record instance and would never be used 100 times in a single GEDCOM.
You asked: "I don't know what FTM 2012 does when it encounters 5.5 tags it doesn't support, I've only read what it doesn't do."
FTM generally drops tags that it does not support, sometimes it reports these dropped tags as errors during inport of a GEDCOM other times it is very quiet.
I hope this answers your question!