The main components of a grid are quality (QG) and yield grade (YG), and whatever carcasses do not fit the grid, or “out-cattle”. The latter are defined as carcasses that reached B-maturity and score slight for marbling or less (sometimes these carcasses are referred to as no-roll), carcasses for which weight falls outside the range stipulated by the formula (often carcass weights ³ 900 lb or weights = 550 lb), and carcasses that display dark ribeyes (dark cutting beef). Quality grade is determined first by age, then by degree of marbling. Age and degree of marbling are estimated at chain speed.
Carcasses determined to be within A (9-30 months) or B maturity (30-42 months) may fall into quality grades widely recognized as Prime, Choice (only average or high Choice for B-maturity), Select (only A maturity), standard, and utility (only B maturity). Within A maturity, carcasses with marbling considered to be slightly or moderately abundant reach Prime, those with marbling considered to be small, modest, or moderate reach Choice, those with marbling considered to be slight reach Select, and those with marbling considered to be traces or devoid reach only Standard grade. Within B maturity, the only change is that carcasses that reach small marbling cannot be graded as Choice, and Select grade was eliminated in 1997 in an effort to reduce tough eating beef.
Therefore, B-maturity carcasses that reach small or slight marbling are graded 2 Standard. Yield grade is an estimate of carcass primal cut yield relative to total carcass weight (these cuts represent a small but highly priced components of the carcass), which varies between 54.6% for YG 1 and 45.4% for YG 5. Factors driving YG up are carcass weight and fat content (fat cover and internal fat); the only factor driving YG down is degree of muscling (area of the ribeye).
Source:http://www.extension.umn.edu/Beef/components/publications/cfr03.pdf