How Do TPG's Grade

I used to belong to NGC collector-society and had 5 coins coupon for every year I renew.
And they state somewhere that at least 3 grader will each come up with grade and the scores will be added/divided to get an average to that coin.

There are limits as to what you can submit for grading with the coupon, but you can pay for the additional charges like variety plus service.

Now, with more time I spend here and reading other articles from numismedia and the likes, I can guess most of what coin will grade, that is with series I am interested in.