Are the races A/B/C or do they race at their USAC categories?

The fields are separated according to collegiate categories. So, there is an A race, a B race, a C race, and so on. Your USA Cycling category does usually correlate to a collegiate category, though, and some of the collegiate categories encompass more than one USA Cycling category. On the road, for example, A's are Category 1, 2 and 3 (but only those with enough points) riders, while the B's are Category 3 and 4 (with enough points) riders. So a Category 3 rider may be fit into either category based on the number of points accrued. For more information, take a look at the collegiate policy document.