Cannon refers to the cannon of the storyline, I.E., what is actually part of the main Course Books storyline. The cannon is very large, and includes the Consectuion Books, Antelope Books, and a variety of novels, novellas, and short stories not in a specific series. The Course Books storyworld is vast and expansive.
There are a series of Alternate Timelines references in the Course Books and other works, these timelines are not considered part of the cannon, even though they are mentioned in the series. There is also the April Fools series of stories which address alternate timelines; these are not mentioned in the Course Books and are thus not cannon, despite being set within the same storyworld.
There are very few completely non-cannonical works which are entirely unrelated to the Course Books storyworld.