The infamous "c word" as it is applied to Star Trek is a joke.
More to the point, it came about in the early 1980s and turned fan against fan. This practice is still going on today. Paramount in turn used it to exploit gullible geeks into buying (or Not buying) various publications.
It goes something like this... GR first comes along and says that only the first two season of TOS are canon and the rest isn't. That includes TAS as not being canon.
Then various TNG episodes and works obviously don't follow this, and why should they? It's all Star Trek. Then the geeks start to appear and begin asking questions about various books being canon. So some king geek invents new words such as Semi-Canon, Pseudo-Canon, and others. Or a Canon Scale. Fans keep arguing. After GR dies, Paramount starts issuing BS about old publications such as blueprints and manuals which were fully authorized by GR, but are now non-canon. This is because they weren't published by Pocket Books but by Bantam and Ballantine. Rival publishers. So in order to maximize their money, Paramount dictates that these are non-canon, despite their obvious influence on Trek films and even Appearance in films. Conversely, Paramount authorizes a few cutaway posters which sell for many times their worth as being Canon because they're making money on it. Their research is piss-poor as are simple spelling of words. But, you know, They're CANON because they're saying so. So fans buy the crud up in droves. Meanwhile a RPG company named FASA which was 100% Paramount licensed and enjoyed by many fans in the 1980s is later called Non-Canon by the same suits because FASA didn't want to pay the increased royalties and what not. So New RPGs are invented which are piss-poor researched and consist of lots of glossy photos and no substance but is called Canon by TPTB because they're licensed... This BS goes on and on, back and forth. Later episodes of spinoff Trek series start using info from TAS episodes--but is TAS canon? Not according to TPTB because of certain licensing issues like the use of Larry Niven's Kzinti who put them there from his own Known Space series into TAS himself but wants to be paid for so much as uttering their name. So TAS is a no-no despite virtually all TOS fans accepting it as a continuation of TOS, at least so far as the stories are concerned. Meanwhile some pipsqueaks who have Titles at Paramount Pictures Trek production offices like Michael Okuda and Rick Sternbach are The only people Authorized to publish anything Treknical for the fans, even if they take the material as half-jokes themselves and constantly contradict themselves because to them it's just a TV show. This automatically cuts out All of the more polished and professional Treknical experts who've published some really splendid works--Out of the Canon category.
While all the above is going on, Pocket Books original Star Trek novels went through various phases of changes including a "disclaimer" section following the copyright about the novel being strictly the author's interpretation of the Trek universe and should not be taken as gospel, blah blah blah... These things change as often as cover artists and fonts, but that doesn't stop some fans from noticing it and thinking that These books are Less Canon than other novels. So a big firefight erupts over that. Taken all together we have fans fighting over fans over what is more "real" in Trek and what is less "real" in Trek. This affects the sales of books and merchandise and makes Star Trek fans look like total assholes.
My take is that Canon is a kid's word and I avoid it like the plague. I take All of Star Trek in as a vast multiverse of Treks. Some things fit better than others. Those that fit the best I accept, even if it comes from some B novel written in the 1970s or some long-forgotten blueprint package or RPG. Even if someone new comes along which everyone is quick to accept, I will scrutinize it and see how well it fits into My own personal perspective of Trek, regardless of who or what came up with it. A good example is the atrocious publication called Star Trek Star Charts which contradicts a much older and more plausible Star Trek Maps, and is in fact a cheap rewrite of the former, shrinking UFP space down to fit the ridiculous demands of Star Trek: Enterprise. In it, Procyon is now the sun which Andor (or Andoria) orbits. For 40 years virtually every Treknical reference placed Andor in orbit of Epsilon Indii, but this new author had to be different or careless. I disregard the new and accept the old in this instance.