James and Steve's book list Rarity 6 and above as RARE. R-6 is one with known survivors of 31-75. Higher rarity codes are named Very Rare, Extremely Rare, and Exceedingly Rare. R-10 is called Impossible, with 2-3 known. I would not disagree with this scale and the names assigned, although I agree it is arbitrary. The "Known Survivors" quantity is also very subjective. James is very knowledgable and sees lots of chips. He doesn't have the ability to look into every hoarder's vault, though.

We also need to keep in mind that rarity is a dynamic attribute of a chip. Some of the Horseshow ARODIE chips were RARE, until about a year ago. I suspect they will be R-3 in the new book: Fairly Common.