Where can I find a definition of the airline fare codes? I am mainly interested in codes "L" and "M"

Which airline? An L or M fare would be different on airline A vs airline B.

Usually during the process of booking a ticket you can read the fare rules before making the final decision.

I'm not sure if the info you seek has anything to do with mileage earning but if it does, the way to check that is to actually read the airline's mileage earning rules and see which fares do earn miles. This information will not be in the fare rules.

If it's other info, such as date, routing restrictions then you should be able to pull up the fare rules before the final click.

Try rooting around:

http://www.ityt.com/library/fom-serve/cache/2.html

It looks to be on topic, but the information is not exhaustive.

no definition...economy cabins can have app. 9 fare codes. "y" being the highest, least restrictive. l and m, depending from airline to airline, sounds low...and with restrictions. the fare would be lower. in effect, someone can have paid "y" $$$$ and be sitting next to someone who paid "l" $...

When my TA was studying for his MBA,part of his research centered around the complexity of fare basis codes and classes of service.He noticed that American Airlines uses 25 of the 26 letters of the alphabet for either a part of a fare basis code or a class of service.In contrast, the LCC's use fewer than 10.

AA has first class, business class, competes against international airlines. LCCs compete on domestic routes and in many cases do not have different classes.