It won’t satisfy your curiosity about just what kind of meat you’re eating to learn that the decisions about gradations are, when it comes right down to it, quite subjective.
Government inspectors, the only ones permitted to make such decisions, rate the meat right at the slaughterhouse, an entire cow at a time. The overriding factors are color and amount of marble. The brighter the meat and the more tiny flecks of fat, the higher the grade.
The inspector stamps as Prime the meat he considers of highest quality, after which come the categories of Choice, Good, and Ungraded. No fixed standards exist, however, and decisions vary not only from inspector to inspector but from day to day, depending on how a particular inspector feels.
If he has indigestion from too much steak the night before, it might have some bearing on what lands on your own table.