The term prime rib is often incorrectly used as a label for what is actually a rib roast. Culinarily, the term prime actually refers to the highest USDA beef grade. It's only given to the finest beef, hallmarked by even marbling and a creamy layer of fat. Very little prime beef makes it past the better hotels and restaurants or prestige butchers. The best grade of beef generally found in supermarkets is USDA Choice. Therefore, although "prime rib" is how rib roast is often labeled, chances are that it's USDA Choice beef.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.