It's quite simple actually!
The base of the modified firmware is mentioned on the Firmware page and in the ReadMe file that is in the zip file.
When a new firmware version is given to me, I'll use this as base!
For now I only have access to YJB8223B.
So if you have any other one, you will downgrade the player.
The Different CP's is based on what language you want it to support in file names showned in file browser!
So if you go to firmware page and look under CP1250, you will find that it is suitble for:
English, Polish, Czech, Slovak, Hungarian, Slovene, Croatian, Serbian (Latin script), Romanian, Albanian and German languages!
So if you for example use Swedish or actually any other language in file names it will come out just as garbage to you!
If you use english only you can use any file!
I hope this made you understand a bit more of the CP system!
Other wise go here: http://en.wikipedia.org/wiki/Code_page
scroll down to: Windows (ANSI) code pages
(Sorry for poor spelling)