Short answers, pulled from the story.
Ruth Plumly Thompson wrote The Royal Book of Oz under the direction of Baum's publisher Reilly & Lee. This novel became the first authorized continuation of the Wizard of Oz series published in 1921.
Disney declared all Expanded Universe material published prior to the 25th of April 2014 non-canonical Legends. This decision excluded the first six films and The Clone Wars animated series from the new timeline.
The Simpsons designates its Treehouse of Horror Halloween specials as non-canonical to allow for wild supernatural scenarios. This boundary preserves the integrity of the show's normal continuity.
The estate of the original author authorized Heidi Grows Up in 1938 as an approved sequel. This practice allowed publishers to dictate the canon of a work long after the writer had passed.
Fanon describes plotlines that become so widely accepted within a community that they function as truth for the fans themselves. This phenomenon creates a parallel canon that exists outside the official narrative.