Yamaha Model Name Designations (XJ, FJ, FZ, YZF, etc)

[radare]

In trying to figure out just what to call my FZ-XJ hybrid, I began to wonder; what do the model designations Yamaha uses mean? For instance, what makes an XJ an XJ or an FJR an FJR? I know the number after that is the engine displacement class, but what does the first part mean? What is the difference between an XS and an XJ that gives you an S instead of a J, but that's similar enough to both be X's?

Here's the model designation list from Wikipedia. Let's see if we can get to the bottom of this: