When a fanbase develops as much as that of Sailor Moon, some elements get analyzed and argued so many times that they become fan canon, sometimes called fanon. If something is not explicitly stated by canon but is not contradicted by it either, it can become fan canon. Romances and ideas about the Silver Millennium most often seem to become fan canon.
Fan canon is made by different interpretations of the source material, so elements crafted completely by fans (e.g. Otaku Senshi) are not a part of fan canon.