NTRP Ratings
The USTA uses NTRP ratings or more commonly NTRPs to rate the players based on skill level. The NTRPs range from the beginner 1.5 with .5 increments all the way up to 7.0 or world-class players. The NTRPs are used in the leagues for equal levels and tournaments. The most common NTRPs are 3.5 and 4.0. At the end of a calendar year, the USTA reveals online what rating everybody in the program is. Rating depends on how many matches you've won, what the score was, and if it was a singles or doubles match. If you are joining for the first year, a guide is given to you to self rate. All of the matches played in the spring season are taken into account of what ranking you are.
http://www.eastern.usta.com/For-Players/20016_USTA_Adult_Senior_Leagues__Ratings/
Read more about this topic: United States Tennis Association