What to add or not to add?

[Ushio]

I've been searching, but I didn't see anything that answers about album types. I know well that this website is meant for video game music and anime music. However, there are albums that are unrelated to that. Some of those are allowed/kept, some are deleted.
I would really like a good explanation for why that's the case. Things like M3-28 I have no idea what that even is. There are albums here with that, and some are deleted for that reason alone.
Wouldn't there be an alternate database that keeps those albums?