Here's what the USCF says about categories. Evidently women only go to cat 4 - probably because there aren't too many women racing.


Each USCF rider is assigned to one of five numbered Categories (Women are assigned to one of four), depending on experience and race results. New members are automatically assigned to Category 5 for Men and Category 4 for Women. Many local recreational riders never seek to advance beyone Category 3. Riders can upgrade to the lower-numbered, more competitive categories based on a combination of experience and race results. In general, categories are as follows:

Category 1: National Caliber atheletes who often compete in the same races as professionals.
Category 2: Advanced riders with several successful seasons of experience.
Category 3: Intermediate riders with one to three years of experience.
Category 4: Novice Riders in their first or second seasons.
Category 5: New Licensees and 1-day licencees.