Otherwise known as "class of service" bonus, earned by flying high-priced fares, e.g. full-fare economy (Y/B), business (C/D/Z), or first (F/A/P).

BTW, welcome to FT. I would highly recommend reading the FAQ stickied at the top of this forum, and becoming familiar with the search utility, as it will save time and help answer your questions before you even have them.