Masters is an age group, not a category. If no category is listed, then it is open-category, meaning that anyone, from Cat. 5 to Cat. 1, who is within the age limit given may enter (including riders with one-day licenses). The age groups are entirely up to the promoter, except for championships. Sometimes masters age groups are also divided up by category. For example, Master 35+ Cat. 1-3, where you would have to be both a Cat. 3 or better and 35 or over. Yes, masters often are eligible for more than one race at the same event, and many of them will race their master's age group event and then turn around and race their category event later in the day. In fact, the masters on our team will often enter the Cat. 1,2,3 race, for example, simply to help help one of the younger Cat. 3 riders. They'll do a lot of work early in the race so that their younger teammate can conserve energy, and then, since they never intended to finish, they'll drop out when they get too burned out to be of any more help.
Promoters will often combine categories if turnout is expected to be relatively low. They will typically offer a race for Cat. 1-3, for example in areas where there aren't a lot of riders above Cat.3. I have also seen events where there is one race for Cat. 4-5, and also an event for Cat. 5 only. That gives the aspiring Cat. 5 guys a chance to see how they fare racing against some Cat. 4s before they actually upgrade. Again, all of these different groupings are up to the promoter except in official championship events in which cases they are generally set (see the championships section of the USCF rulebook at
www.usacycling.org).