It's for characters that are not associated with any known copyright.
Posted under General
One gripe I have is that the vast majority of images that have no copyright tag of any kind are probably also original characters, even though there's no official confirmation from the artist that it's original.
Things have become somewhat easier since I figured out how to do a copyrighttagcount:0 search. But I cannot combine this with Original, so I have to run two searches with whatever tag strikes my fancy: 'tag original' and 'tag copyrighttagcount:0' and that gets rather messy.
I wish there were an easier way to search through images without established characters.
If I'm reading the DB cheat sheet correctly, you should be able to find posts with original or copytag:0 by using tildes (~).
Example: ~original ~copyrighttagcount:0, plus your other tags