Tamura Jones has a couple of more articles out delving into the intricacies of GEDCOM files, this time covering the GEDCOM RIN. RIN, or Record Identification Number, is a unique number assigned to the record by the source system, as defined by the GEDCOM 5.5 standard (released in 1996). As noted in the article, it’s not necessarily a record meant for humans, rather it’s meant for working/communicating between two systems, and synchronizing the data between the systems.
Ironically, while FamilySearch handled both the GEDCOM standards as well as their own desktop genealogical program, Personal Ancestral File (PAF), and while PAF 5.2.18 (2002) exports GEDCOM 5.5.1 files, it does not support the RIN record. At the same time it supports another type of RIN – individual (INDI) record and MRIN – family (FAM) records.
The second article is a very extensive look at just the GEDCOM RIN side of things, along with discussions about various genealogical programs (and services such as Ancestry.com), and how the RIN record is actually “unfit for its intended purpose.”
Tamura has a much better understanding of all of this, and you should just go read his articles: