GIRL you must have a marbling score of abundant 99 because you're a PRIME CUT
Beef cuts are ranked based on quality grades. Quality grades are determined on marbling and maturity, but marbling is the primary driver. The marbling, or intramuscular fat, are broken into different categories that relate to a quality grade. Because fat is responsible for flavor and tenderness, the more fat usually means a better the eating experience (and the more expensive a cut is). A steak graded prime, the most valuable quality grade, will have the most marbling. [BUT this should not be confused with the cut Prime Rib, which does not necessarily mean it will have a prime quality]
Marbling scores are broken down into 8 categories (from a scale of 0-99) that correlate with 4 quality grades for young cattle:
Abundant--High Prime
Moderately Abundant--Average Prime
Slightly Abundant--Low Prime
Moderate--High Choice
Modest--Average Choice
Small--Low Choice
Slight (50-99)--High Select
Slight (0-49)--Low Select
Traces--Standard
There are other grades for older cattle, but they will usually not be offered at the retail level due to off flavors and tenderness issues (due to collagen cross-linking). The Select quality grade is special though, because it is reserved for cattle between 9-30 months of age with Slight marbling score.