Grading for lamb quality is voluntary. A meat processing plant may request to have its lamb graded for quality based on traits such as tenderness, juiciness, and flavor. USDA-graded lamb sold at the retail level is Prime, Choice, and Good. Lower grades (Utility and Cull) are mainly ground or used in processed meat products. 

Lamb quality grades take into consideration maturity (lamb, yearling mutton, and mutton), conformation, and the palatability-indicating characteristics, such as fat streaking within the flank and firmness of the lean. Most of the graded lamb sold in supermarkets is USDA Choice; 80% of the American lamb supply is USDA Prime or USDA Choice. The protein, vitamin, and mineral content of lamb are similar in all grades.

Source: usda.gov