Original x Copyright

When do "original manga" or similar works get a copyright tag of their own? I don't know how to better explain this question. I guess this happens only or mostly to doujinshis; some works are tagged as orginal while others have their own tag, and I can't tell if it's because of popularity/well-knownness (does this word even exist), number of volumes or whatever, or it's just not really regulated well. Sometimes they even have original and the copyright tag at the same time, is this a good solution?

I don't know if we have any rules about this, but it might make sense to at least require that the the original universe in question has a name, or else creating a copyright tag for it would be difficult (we'd have to resort to something like artistname's_original_universe).

Other than that I'd ask "Would having a tag for this help searching?" If the original universe in question is limited to a one-shot comic with no fan art or sequels, then creating a tag for it doesn't have much use since people could just go to the pool containing that single comic.

Supplementary art by other artists is a good rule of thumb. If you can get everything within that universe by searching the artist's name + original there's no point in giving it it's own tag.