An indication of the number of chips known
The rating comes from Campiglia and Wells "Official U.S. Casino Chip Price Guide". An R-8 means the authors think that there are only 8-15 chips known in collections. A new edition of this price guide is due out the first of next month - you can search the board for James Capiglia's post announcing the availability and how to order them. Others, including CT Rodgers are also selling them.