"Prime" is the top grade. "Choice" is the next grade below Prime. "Select" is the lowest of the top three grades.
A prime grade standing rib roast will have more intramuscular fat. That's the little white lines of fat within the red meat. Lower grades will have less intramuscular fat.
More intramuscular fat is a good thing because it results in a more tender and juicier roast.
External fat is what is around the outside of the red meat.
Whether a standing rib roast has more or less external fat is more dependent on which end of the standing rib you got your roast from. One end has more fat and the other end has less.
You can trim of excess external fat but leave some fat all around to keep the roast from drying out during cooking.
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