This is from a long post by Jugggy da Beerman,a comp cook,from Ray Basso's forum.
A lot is technical about beef carcass contest grading,but this paragraph speaks to our questions about what we may see stamped on beef at the grocery.
Beef quality grades are determined by two factors: marbling and maturity. Marbling is the small flecks of fat found within the muscle. Marbling helps keep the meat juicy, adds flavor and has a slight correlation to tenderness of the meat. Maturity is based on characteristics of the carcass, such as the amount of bone cartilage that has turned to hard bone, but relates to the actual age of the live animal. However, in carcass shows, maturity has minimal effects on rankings as nearly all animals are in the "young" or "A " maturity classification, which includes beef animals between 9 and 30 months of age. When animals are all in the same maturity class, they all are eligible for the same quality grades.
The quality grades for "A" maturity animals are Prime, Choice, Select and Standard. Marbling is divided into different degrees or ranges from Abundant to Practically Devoid. Prime is considered the highest quality grade and has the most marbling. Nationwide, only about 2% of carcasses grade Prime. Choice is the next highest quality grade and is divided into thirds: High Choice, Average Choice and Low Choice. The Select grade is divided into High Select and Low Select. Standard grade carcasses have the least marbling. Each quality grade has a specific degree of marbling associated with it, which is usually listed adjacent to quality grade on carcass data.
In many carcass contests, carcasses must grade Choice or Prime to be eligible for the top rankings. This is largely based on industry standards that discount carcasses below a Low Choice quality grade. Carcasses that have an Average Choice or higher quality grade often receive a monetary premium, and may receive some advantage in the ranking system. Because of the negative effects on juiciness and flavor associated with the lack of marbling in Standard carcasses, these carcasses usually are heavily discounted, and are in the lowest rankings in a carcass contest. A "No Roll" carcass is one that is not quality graded, usually because it will be a relatively low quality grade, either Select or Standard.
"Dark cutters" are another quality classification that will be in the lowest rankings of a carcass contest. "Dark cutters" have lean that is dark red to almost black in color with a sticky or gummy texture. This condition often occurs in cattle that have been stressed for a relatively long period of time just before slaughter. Dark cutters are safe to eat and their taste and tenderness is usually not negatively affected. However, consumer acceptability and carcass value are reduced.
"Bullocks" are carcasses that often are not quality graded and receive a NoRoll classification. Bullock carcasses are those that are in the "A" maturity classification, but exhibit characteristics similar to bull carcasses. These characteristics include darker and coarser textured lean, a heavy crest (neck), and one muscle, commonly called the "jump muscle," in the round that is highly developed.