Fanon assumed as canonEdit
Some fanon is considered "community wide". It applies to certain "facts" that may have been accepted as a truth by a large number of fans, although not necessarily true, and thus either replaces an established canonical fact in the minds of those fans, or fills a plot-hole.
One example of this form of fanon is Sulu's first name. Hikaru was used for years in fan fiction and books before finally being "canonized" in ST IV.
Fanon made official canonEdit
Fanon is occasionally adopted by authors, such as the Franz Joseph Enterprise deck places being seen on screen in Star Trek: The Motion Picture, and so becomes canonical. Fanon which is accepted officially as part of the canon ceases to be fanon.
Fanon can also take the form of personal beliefs held by individuals. In this case, an individual may create an expansive backstory, possibly a fan fiction, which he or she accepts as "true."