It is a little bit of both Mark. I think it goes back to the original appearance. If the original appearance of a character is public domain then the character may be used even if it is still trademarked. You may not however put the character or the trademarked name on a cover in such a way that it might confuse someone that it is by the trademark holder. Charlton Action Heroes would be an example. Since their original appearances are public domain then anyone could make a comic with the characters inside.