Cloth

 J - A light weight, flexible Egyptian cotton cloth
JF - A light weight, very flexible Egyptian cotton cloth
X - A heavy, stiff Egyptian cotton cloth
XF - A heavy, yet flexible Egyptian cotton cloth
YX - A lighter weight polyester backing
YY - A very heavy, stiff polyester backing

Fiber - A very hard, strong, coated abrasive backing material consisting of multiple plies of chemically-impregnated paper. Used primarily for disc products.

Paper

A - A very light weight paper - typically for sheet use only or light PSA or hook & loop disc usage
B - A light weight paper - typically for sheet use only or light PSA or hook & loop disc usage
C - A medium weight paper - typically for sheet use only or light PSA or hook & loop disc usage
D - A medium to heavy weight paper - typically for sheet use only or PSA or hook & loop disc usage 
E - A heavy weight paper - typically for stroke or wide belt sanding
F - A very heavy weight paper - typically for stroke or wide belt sanding