At ANs (and Adult Sectionals), events with a Championship designation mean that you must qualify from Sectionals to compete. All of the age groups are combined, and the top four from each Section qualify. These levels are Championship Gold, Championship Masters Intermediate/Novice, and Championship Masters Junior/Senior.
The non-Championship Masters events are open events. This means that you skate at the level you are eligible for, and you do not have to qualify from Sectionals. Depending on number of entries, these are divided by age group. So Masters Intermediate means that anyone eligible for that event may enter, at their appropriate age category. The levels are Master Intermediate, Masters Novice, Masters Junior and Masters Senior.
Actually, if you read the announcement for this year's ANs, it has links to the current rulebook and the explanations of the events, which may also help.
http://www.2014usadultnationalchampionships.com/wp-content/uploads/2013/12/2014-USAC-FinalAnnoucement.pdf