The only "standard" I know of is ANSI/IEEE 315-1975 with Clause 22.4 being the "standard list". Although IEEE 315 is no longer ANSI it is still the only viable list of class designation letters. The Europeans, under ISO and IEC have their own system but I don't think you want to go there. The IPC has published a standard, non ANSI, that includes class designation letters in an appendix in which they refer to IEEE 315 and add some other class letters. However, the class letters of parts they add are already covered by class letters of IEEE 315. I guess they can't read or think they are in their own little world.