There are four grades for Cheddar cheese: U.S. Grades AA, A, B, and C. As with butter, all grades may be used in the wholesale trade, but only the top grade is used at the retail level. To rate the top grade, the cheese must have a consistently fine Cheddar flavor. In addition, there are grades for Swiss cheese, Emmentaler, Colby, Monterey (Monterey Jack), and Bulk American cheese for manufacturing.
http://www.ams.usda.gov