Q. Can you explain what the numbers and letters mean on the fare basis code.
I have a flight booked on Northwest with the code T7RV3N. Can you
decode that? Is there a table that lists the various codes that I can
refer to for future flights?

A. Well, the first letter is the fare class, T for example is a really cheap non-refundable fare. Y means full fare economy, and F is full fare first class. M and B are discounted but more flexible economy class fares. 7 refers to the advance purchase requirement (7 days); the 3 as in 3N means there's a 3 night minimum stay.

TR21M1SN would mean a non-refundable coach ticket with a 21 nigh advance purchase and a 1 night min. stay

As far as we know, there's no hard and fast set of rules as to how the airlines come up with these codes, and they vary from airline to airline.