Generally? When would they be?

As I said on Discord, I feel these two parts are problematic.
I get that the original focus of the site was for game music, which is (or at least was) mostly BGM, but this whole topic is about the expanding of scope that has happened since then. I'm not sure why a composer role is given that much more importance than a lyricist or performer role now. Even if we just consider the composing role, someone who primarily composed Category 1 songs wouldn't be eligible for Category 3, but composing one 15-second little bit of BGM would suddenly qualify them.
I also understand that just singing one song for a game or anime shouldn't mean an otherwise unrelated artist should get their whole discography on the site, as that would include far too much general J-Pop. But IMHO other works by composers, lyricists, and even performers, who are largely known for their Category 1 work should qualify, even if they haven't composed any BGM.
I feel a subjective "ratio" could be a better system after all for these artists, instead of (or perhaps along with) the BGM qualifier.
And what of instrumental versions of vocal songs, that are used as BGM in anime? That sounds like BGM to me, so should qualify, but really the only way to know if an instrumental track was used in such a way would be to watch the anime.

edit:
And as an example of kind of the opposite...
Koji Makaino has composed BGM for several anime, and so should fit in Category 3, allowing anything else he's worked on to be included... But he has also composed hundreds of non-Category-1 songs, which according to the proposal should now be fair game for the site. While I personally wouldn't mind seeing all that here, I expect some others would.