When most people think of "class" in terms of airplane travel, they consider the class of service, such as first, business or coach. But airlines organize classes with a much more complex structure, with letters representing a "fare class," not only a cabin. These letters are often tied to a particular class of service, but each cabin will have multiple fare classes assigned, with each letter representing the price a passenger paid for their seat.
If you're just getting started with miles and points, it's a bit early to begin obsessing over fare classes, but if not now, you're going to want to learn about them at some point.
To make things even more confusing, different airlines use different letters to represent different levels. K, a deep-discount fare class on United, may be tied to a much pricier bucket on another carrier. But across the board, most airlines use F to represent a full-fare (very expensive) first class ticket, with J assigned to full-fare business and Y linked to full-fare economy. From there, things get very, very confusing.
Fare classes are one reason why one passenger may have paid significantly more for a flight than the customer sitting next to him. Some companies require their employees to purchase refundable (full-fare) tickets, but even some leisure travelers pay more for the same seat than others. Typically, airlines release a certain number of deep-discount fares for each flight.
Once those are sold, the ticketing system moves up the alphabet to the next class. Similarly, if only two deep-discount tickets are available and you search for four (to keep your family on a single reservation), the system will return the first class with four available seats. For this reason, you may save money by booking seats individually after you locate a flight that has enough room to accommodate your group.
Before you book a ticket, you should also match the available fare class to your frequent flier program's earning chart. In some cases, your fare class may not be eligible to earn miles, but nearly all paid fares with US-based airlines earn at least one mile per mile flown. International airlines may not award you any mileage with deep-discount tickets, however, and your US-based program may also not issue award miles for certain fare classes on partner flights, even if the flight is operated as a code-share. For example, if you book a flight through United that's operated by Austrian, even though the fare class would qualify for mileage if it were a United flight, your frequent flier program will credit you based on the operating airline.
Fare classes are also used to represent award ticket availability. Often, if a deep-discount fare isn't available for a given flight, you won't be able to book an award seat at the lowest redemption level. Business and first class tickets are handled in the same way, so if the only available first-class fare costs $15,000 roundtrip for a route that carries a $10,000 discounted first-class fare, you may have a hard time booking an award. Advanced users can use fare classes to their advantage with a tool like ExpertFlyer.com.
There, you can see available classes for many flights, making it simple to search for award seats several days at a time.