As it's name implies, this tag is used if a copyright from which a character comes from is unknown.
If it is known that a character doesn't come from an established copyright, then use the original tag. See the wiki page for original for a list of Japanese tags that indicate an original character.
http://www.animecharactersdatabase.com/ - If the name of a character is known, You can use this site to find the copyright if they come from an anime or game.
