The NRA has established a set of standards for classifying shooters. This is based on the average of your last 3 matches in NRA "Approved" or "Registered" matches. The Match program will tell you if a particular match is an approved or registered one. Practice matches are not, and State and Regional Championships are. If you have not fired in 3 approved matches, you can use a "Temporary" classification based on your running average of matches you have fires, practice and approved. After you have fired your 1st match, ask the match director for a temporary classification book, they are free. If you have never fired in a High Power match before, you will compete as an "Unclassified Master". If you can not show proof of classification, you will compete with the masters.
Once you have a classification, you will compete against other shooters in the same classification. If there are not enough shooters in your class, you may be combined with the next classification up.
Here are the classifications based on a percentage of the possible total score
Marksman Less than 84%
Sharpshooter 84% to 88.9%
Expert 89% to 93.9%
Master 94% to 96.9%
High Master 97%