There’s one characteristic that Prime beef has that other steaks don’t: a whole lot of marbling (intramuscular fat) between the 12th and 13th ribs (the area inspected by the grader). The more marbling, the more tender and juicy the final product will be.
The three main grades of beef that inspectors separate carcasses into are Prime, Choice, and Select. Prime beef comes from young, well-fed cattle and has abundant marbling. Choice beef is also high-quality, but has less marbling than Prime. Select beef isn’t as well-marbled as Choice, and is best when marinated or braised.
Only about 3 percent of all beef is Prime, about 70 percent of all beef is Choice, and about 20 percent is Select. The remaining beef falls into five lower categories: Standard, Commercial, Utility, Cutter, and Canner. Standard beef comes from younger cows but is “practically devoid” of marbling; Commercial beef has a “moderate” amount of marbling but comes from older cows (and these are the lowest two grades you’ll find at the supermarket).