One of the most common areas of confusion relate to the bike pit, so take a look at those rules in particular. Generally each pit will be accessable from two areas on the course (double sided pit). You are not allowed to cut the course or go backward to get to the pit. Riding through the pit without changing a bike, wheel or a repair is not permitted and you must exit on the same side of the course that you entered. You are responsible for your spare equip, but generally there will be some mechanical assistance available to get you rolling again.
You will note that we have a Single Speed category. These are bikes with a single front chainring and single rear sproket or some mechanism to prevent shifting. Making thoughtful gear selection based on the course is important to be as efficient as possible. Your pit bike or spare wheel might have a different gear ratio and that is allowed per our USAC national rules guru.