Jake's MSPA Wiki article is imo the "objective" answer in terms of the idea of labelling of canon as defined by how it's classified by "word of God" Hussie or previously accepted Canon Authority announcing it's level of "authenticity."

Skitis still has a more functional definition in an active fandom sense, of canon being relational and based on the compounding acceptance of works (if you are working in the "canon" of E, you have accepted the canon of A->B->D->E (skipping C to underline that not all are linear or require everything that came out chronologically before it)).

Oasis is correct in regards to it all being arbitrary. Like the idea of certain things being "dubiously canon" is universal for IP/stories that have multiple writers/creators who may have different ideas than the "true originator(s)." As Rozie mentioned (also: HEY LONG TIME NO SEE!!!!!!!!!!) Star Wars has the expanded universe which could have been generally agreed upon as canon until a "more canon" form of the mainline movies dismissed it, thus making it apocrypha. It would require a more Canon Authority to then return to it and elevate it into being considered canon material.

But this is also all based on fandom acceptance. And I used fandom acceptance here to also include creators. For example, if JJ Abrams had been obsessed with the expanded universe of Star Wars, he almost definitely would have "canonized" it by referencing it in his movies. If he becomes obsessed with it after making movies that might have contradicted them and made them seem apocrypha, he might even still be able to say "actually those are canon," retroactively if he still retained enough Canon Authority (if people still liked him enough).

This leads into Homestuck: the main reason there's still arguments about levels of canon is because people don't(/do) like it more than it's been labelled and/or are unconvinced on the Canon Authority who has defined it. The reason JJ Abrams had more Canon Authority to dismiss the expanded universe was because people watch movies more than tv shows or books or whatever. What the fandom remembers people think of as canon. Which is why fanon is so scary. When the Homestuck cartoon comes out people will be forced to come to terms with the absurdity of arguing over "canon" as so high stakes when the cartoon comes out and if anything, more people might end up watching it than reading the actual comic.