We may be looking at two different sets of "classes" here.

1. Cabin class - most mainline airplanes have two or three classes; coach, [business], and first. The normal designator for coach class is Y. Business class is commonly C or J. First class is almost universally F.

Besides this, there are subclasses, like Z class may be a First Class seat on an award ticket and things like that.

2. The second class type is fare class. This is somewhat loosely coupled to the cabin class, in that a full-fare Y tickets will generally have a fare class of "Y"; similar with business and first.

However, there are a myriad of other ticket class available, usually reflecting different fares, restrictions, etc., and usually all resulting in a seat in coach.

Thus, we find USAir has classes M,H,K,V, and probably a bunch of others. They are often (but not always) different prices and with different restrictions (Saturday night stay, 7 or 21 day advance purchase, upgradeable or not, etc.). It's this type of class profusion which leads to a lot of customer confusion.

And if I'm not mistaken you're probably a bit confused at this point, too.