Classes besides F, C, and Y are usually dependent on the airplane and simply refer to fare classes within each cabin. Each airline has its own set of fare classes. For example, if you were to run a search at itn.net under expert mode it would show you how many seats at each fare class are available. Not always, however, are you able to get the lowest fare class, but the lower the class goes, the more restrictive it is. There may be one or two fare classes of F or C class, but many airlines have 10 or so fare classes of Y class to satisfy every possible restriction possibility: upgrade, changes, stopovers, standby, advance-purchase requirement, refundability, travel dates, blackout dates, minimum/maximum stay, etc. Hope this answers your question. If you are wondering about a specific airline, you can ask in that airlines forum about different fare classes and an expert of that airline can tell you everything about it. (keep in mind, though, that fare classes also revolve around city-pairs and availability, not to mention travel dates)