Just because its not listed in the rules, doesn't mean its still not a rule.
Mainstream fictional content ONLY (ie video games, tv shows, literature, western animation, web animation, web original (in some cases), anime/manga, light novels, visual novels, comic books, tabletop games)
Those are all considered valid content. A commercial is nothing more than an Ad that is nothing more than gag and not meant to be taken seriously.
The same with songs.
If its not from something like whats listed above, then its not considered a valid user or example.