From all of the previous statements, it seems that the question of where the cuts of beef for "Prime Rib" or "Rib Eye" come from has been answered: They're both from the same cut & basically the same thing (the "Prime" in Prime Rib coming from the word "Primal Cut").
The other question that has been bandied about seems to be "What is considered to be "Prime Quality" beef?
It's been a few years, but the last time I looked, Beef grades are BASICALLY (for home & restaurant use) categorized by the USDA into (3) Grades (in raw form, at time of processing), based upon several factors, including (but not limited to) texture, fat marbling, feeding & genetic history (which breed) of the critter.
1): PRIME GRADE: The top 10% (in quality) of beef on the market. This is what you pay BIG bucks for in high-end restaurants & specialty butcher shops. Best flavor & tenderness.
2): CHOICE GRADE: The middle range, 70-90% (in quality) of market beef. This is what you normally (make sure to check) get in the local super market. Less flavor & tenderness than PRIME.
3): SELECT GRADE: The lower range, 50-70% (in quality) of market beef. Less flavor & tenderness than CHOICE.
There are other, lesser grades used for other purposes.
There are other processes involved (Dry or Wet Aging, etc.) to enhance the final product, but they don't have anything to do with the original USDA Grade.
My percentages may be currently off (like I said, it's been a while) but that's the basic info on USDA graded beef (unless the USDA came out with some different standard grades when I wasn't looking).