USAC-sanctioning means that all ASP racers must hold a USAC license, which you can either pay for annually ($60) or race-by-race ("one-day", $10 per event). I can see all of you reaching for your slide rules; an annual license pays for itself after six races.
When you purchase an annual license, you have to choose the blue pill (Road/Track/CX) or red pill (Mountain Bike/CX), or you can pay an extra $30 and get both pills (Road/Track/Mountain Bike/CX). No matter which you choose, you still get CX!
When you first purchase a license, whether annual or one-day, you are assigned to CX category 4. If you're the ambitious sort and want some day to upgrade to cat 3, you'll want an annual license because (1) your race results accumulate and are tracked by your license number and (2) you can't upgrade to cat 3 without an annual license. On the other hand, there are many people who happily do a few races per year on one-day licenses, and that's cool too.
Clear as mud? Awesome. Let's race.
[PS: Road cat 2 or MTB cat 1 racers can automatically request an upgrade to CX cat 3. http://www.usacycling.org/forms/USAC_rulebook-1.pdf]