I understand it has to do with the marbling of the meat.  Does the grading system apply to the entire cow, as in any cut from this cow is considered prime/choice/select?  Or is it done by the portion of meat cut; could the same cow produce both prime meat and sub-select meat?  And who came up with prime-choice-select instead of a simple letter grading system? 
The grade is by whole carcass.  http://en.wikipedia.org/wiki/Beef#USDA_beef_grades  History of beef grading in the US  http://meat.tamu.edu/meat-grading-history/ 