Rider's categories are based on last year's results.
Because there is a lack of cross over between road or mountain bike racing and cyclocross, racers who are new to cyclocross MUST register as CX 4 male or CX 4 female (race in the C event). If you are too fast for this event you will be moved up after a couple of races. Riders who are extremely strong in road (very strong in cat 2/3 or riding away from the whole field in cat 4) or mountain bike (elite or expert men level) may apply to the cross committee to start in CX 1/2 or CX 3.
Note: The online registration system cannot tell which category you are allowed to race in. If you are a CX 4 rider, but register for CX 3 online you are still expected to show up for the CX 4 race. If you miss the CX 4 race you will be categorized as 'Did Not Start' and will forfeit your race fee. You will not be allowed to take part in the CX 3 race.
The 2013 rider category list will be posted asap.
For more information please see this post and and this one.