Most people that play USTA League plan in a men's/women's league, but many also play in a mixed doubles league. There are some that only play mixed though, and for these, the NTRP ratings are calculated a little differently.
If you play in at least three men's/women's matches, your rating will be calculated based on only those and your mixed results aren't factored in. However, if you only play mixed, you will be given a Mixed Exclusive (M) rating.
A mixed exclusive rating is used only for mixed leagues, and if you have one and decide to play a men's/women's league, you have to self-rate for that league.
So, these M ratings are a bit unique as they would seemingly have to be calculated for everyone who plays mixed in case they don't play men's/women's, but then they aren't used. In theory, a player could be a 4.0C (men's/women's rating) but a 3.5M based on their mixed matches if they hadn't played men's/women's.
I've toyed with calculating M ratings for awhile now, but believe I have something working reasonably well now, so if you only play mixed, or play both mixed and men's/women's and just want to know what your M rating would be, I can now calculate these as part of my Estimated Dynamic NTRP Rating Reports. If you are interested in a report, contact me.