I found this article interesting. I think those who fly a lot will want to be aware of these codes.

NEW YORK – There are a few bits of information to pay close attention to on an airline ticket: the flight number, gate number and boarding time. Fare basis code? Not a common concern.

But the single-letter code can make a big difference in some parts of the travel experience, even though most passengers don't pay any attention. A fare basis code further divides passengers into classes based on how much they paid and how far out they booked. There are about a dozen in coach class alone.

When you're on the plane, there's no difference in service between a passenger who has a "Y" or "Q" — a full-fare and a discounted ticket — if you're both in coach. But the codes are still important: Some indicate your trip isn't eligible for frequent-flier miles or an upgrade; others tell a ticket agent where to rank you on a standby list.

Deciphering the code

The letters airlines assign to certain levels of coach can vary widely, but a couple are universal. "Y" class is a widespread denotation for the highest class in coach among most major airlines, according to Jami Counter, senior director of TripAdvisor Flights and a former pricing strategist at American Airlines. These tickets are usually fully refundable, last-minute coach fares purchased mainly by business travelers. They're the most expensive tickets, but they have the most flexibility.

Some others that are generally used among the airlines: "J" or "C" usually indicate business class. "F" and "P" denote first class or premium.

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Airline codes have been around 15+ years, AA started this type of catergorizing in the deregulation days of the industry to show different airfares. The airlines do their yield management on a minute by minute basis and fare codes change constantly. On any given day 150,000 fares change all based on yields on any given flight on any given day. Fly high market routes, the lower the fares, fly low market routes the higher the fare. You can buy you ticket literally seconds difference from another person and pay a different fare, yet you all sit in coach and you all have a fare basis code.