We comic book fans are very resistant to placing comics in Box Four ("it's just a story") and very inclined to treating all our beloved characters as inhabiting Box Three ("it's an actual universe which just happens not to exist".)
There is no shame in this. It can be great fun. Some Sherlock Holmes fans like to pretend that the stories really are Doctor Watson's write-ups of real cases which a real detective really solved. It's a fine scholarly game and it's been going on for more than a hundred years. I myself had a great time pretending that the early issues of Spider-Man were historical texts: looking at maps of New York and Greyhound bus-timetables, checking up what things cost in the 1960s and working out, to within a few days either way, the precise date on which Spider-Man was trapped under the wreckage of Doctor Octopus's underwater base.
Sometimes this illuminated the story in interesting ways; usually it didn’t. I did a similar thing with the text of Mark's Gospel and a Google map of the Holy Land, with similarly patchy results. But I knew perfectly well I was playing a game, and quite a silly one. So do all Sherlockians and most comic-book readers. New Testament scholars I would not vouch for.
"How could Spider-Man possibly have web-swung from Forest Hills to Madison Avenue in three minutes" is a perfectly good question. "Maybe he can swing at 200 miles per hour" is one perfectly good answer. (That's presumably why he arrives at the scene of the crime just in time like a streak of light.) "Perhaps he bumped into the Human Torch and hitched a lift" is another perfectly good answer. "I think I will write a piece of fan fiction about what happened while Spidey was in the back seat of Johnny's segment of the Fantasticar" is no more pointless than anything else which human beings spend their time on.
"Stan Lee isn't remotely interested in travel times" is a third perfectly good way of answering the question. It has the virtue of almost certainly being true: Lee sometimes forgets that the Bugle is meant to be in Manhattan and that Peter is meant to lives in Queens. Indeed, he sometimes writes as if New York is a very small village where everyone knows everyone else. But "Stan Lee doesn't care" is a different kind of answer from the other two. Shelockians would say that it was Doylist rather than Watsonian. When you are reading a story, you kind of have to be Watsonian. When you are writing smart essays about it on the Internet, you can afford to be a Doylist. It is probably unwise to be both at the same time.
Most of the time, most readers are quite happy to say "Spider-Man travelled from some place, to some other place, in the amount of time it was narratively appropriate for him to take and not a minute more.” But just occasionally -- as when the Angel seems to be able to fly across the Atlantic in ten minutes, or Shang Chi sees thatched cottages and taverns at the edge of Trafalgar Square -- our ability to suspend disbelief evaporates.
Why can't I say "Spider-Man didn't cross New York in three minutes; that never happened; we can cross that bit out; it's just a story"? Because if I do, then I have admitted the possibility that the Burglar didn't kill Uncle Ben. And the fight with the Sinister Six never happened. And that we can cross out the bit where Doctor Octopus unmasked him. And the underwater base is just a story. If one thing isn't true, then maybe nothing is true. And if we admit that we can no longer play the great game.
This applies to more serious matters as well. If maybe-possibly-perhaps Saint Matthew made up the story of baby Jesus and the star and the wizards to make a theological point then maybe-possibly-perhaps he made up everything else as well. If we admit that Jonah and the Whale is maybe-possibly-perhaps an Imaginary Tale, we might have to entertain the possibility that maybe-possibly-perhaps God is also an Imaginary Tale; in which case there would be no point in reading any of the stories ever again. Unless you think that stories have points in themselves.
This is why there are YouTube videos of angry Americans who believe that dragons exist, dinosaurs do not, and that the rotundity of the earth is an elaborate hoax.
There is probably a very good piece of fan-fiction to be written about how Tolkien discovered the Red Book. I imagine that Ronald would be a kind of literary Indiana Jones, running all around Europe with his faithful companion Jack and his clever kid-sidekick Chris; chased by orc-worshipping occultists and Blue Wizard initiates, collecting precious MSS and bringing them back to the Bodelian where they can be properly translated. The Space Traveller in Out of the Silent Planet is a philologist, fairly obviously meant to be Tolkien, and the book ends with a classic Box Three framing sequence in which C.S Lewis says that he is presenting real events as if they were fictional in order to get humans used to the idea that there are alien angels on Mars. The preface to Lord of the Rings strongly implies that a small number of very well hidden Hobbits and diminished Elves still survive in our own Seventh Age. Tolkien knew Lewis and Lewis met Yeats and Yeats new Crowley and Williams was an initiate of the Order of the Golden Dawn. If I had the slightest talent for fiction, I might try to write it myself.
But Tolkien himself doesn't really go in for that sort of thing. The Red Book is something he takes for granted in order to give himself a viewpoint. The unfinished Book of Lost Tales -- what eventually became the Silmarillion -- did have a frame narrative about an Anglo Saxon sailor who travelled to what was still called Fairyland and learned the history of what were still called the Gnomes. And what became the Second Age of Middle-earth was introduced in an unfinished Time Travel story about a modern-day father and son who had inherited a kind of psychic race memory linking them with ancient Numenor. There may even have been a fortnight when Tolkien and C.S. Lewis intended their various stories to cohere into an Inklingverse. The Merlin of That Hideous Strength claims to know the magical traditions of a place called Numinor [sic]. These framing devices are largely missing from the Hobbit, the Lord of the Rings and the published Silmarillion. Christopher Tolkien infers, correctly I think, that the viewpoint of the Silmarillion is "the tales that Bilbo heard in Rivendell".
But Tolkien always takes it for granted that his stories are to be read as Box Three artefacts. They are not Just Stories: we are to pretend that they are historical fictions based on primary texts. Books about the real world, set in an imaginary time, was one way he explained it. He maintains a -- so to speak -- Watsonian perspective even when he is writing informally or making notes for his own benefit. He never looks at an inconsistency and says "I wrote that wrong, I'll have to change it". He is more inclined to say "There are two distinct traditions" or to write a fresh text explaining how the apparent contradiction arose. Asked by a fan why Gollum thought that the Ring was a birthday present given that Hobbits don’t receive presents on their birthdays he improvised a long riff about the different gift-giving customs among the different tribes.
This maybe why some people find Christopher Tolkien so hard to take. Any illusion that Middle-earth is really, really real is thoroughly dispersed by the Twelve Volume History. What we thought of as fixed chronology turns into something contingent and unstable; a set of ideas that Tolkien was constantly changing and erasing and literally overwriting. Your respect for Tolkien the artist increases. Your secondary belief in Middle-earth takes quite a battering.
The trailers for the forthcoming Second Age TV series appear to indicate that there will be elves of colour and possibly Asian hobbits. The racists are out in force, saying that this is not true to Tolkien's vision. To which the only possible answer is "Which vision?"
There is a certain species of fan who feels that a story only matters in so far as it can be located in Box Three. Watsonian readings are the only ones which matter. A story is part of a sub-created world or else it is nothing. Marvel Comics are there to provide us with information about what happened in the Marvel Universe as opposed to entertaining us with unlikely yarns about people who wear their underwear on the outside.
The Story of Superman Red and Superman Blue is, by common consent, delightful. What If The Avengers Had Fought The Kree/Skrull War Without Rick Jones is fairly obscure. But "being delightful" is not what puts a story in Box Three. Nor is self-consistency, good-story-telling or having something to say about the real world. A story goes into Box Three if the writer or the editor or the show runner or J.K Rowling says that it can go into Box Three.
In fandom, as in religion, that is pretty much what Canon means: a text which some person in authority has approved of.
Why is that book part of the Canon?
Because the Canon-Keeper says so.
Who is the Canon-Keeper?
The person who decides which books are part of the Canon.
Only the showrunner, or the editor, or the Holcron Keeper can move a text from Box Four to Box Three. And once they have done so, they cannot be gainsaid. A story really happened if Stan or Roy or Mark or George or Kathleen says it really happened. No other considerations apply. The silly in-joke about the eleven faces of Doctor Who in Brain of Morbius is irrevocable canon because Chris Chibnall says so.
To believe in Canon is to believe in the absolute primacy of Authorial Intention. Your job as reader is to read the text in the way that the author tells you to read it.
In Chaucer's English, the word Author and Authority were the same word.
I could read the 1993 Death of Superman story as if it were an imaginary tale, if I wanted to. And I could read the 1965 imaginary story with the same title as if it was canonical. There are as many head canons as there are heads. Frank Miller once half-seriously remarked that so far as he was concerned, after Daredevil 191 Matt Murdoch went home and quit crime fighting: all the subsequent episodes were "imaginary". I myself have said that I think that Steve Ditko intended The Final Chapter to be the end of the story of Spider-Man: that having purged himself of his guilt, Peter Parker quit being a superhero and concentrated on taking care of Aunt May and studying science. But a label which says "This is an imaginary story..." is a pretty clear instruction from the writer that we readers are not allowed to treat the story as canonical. A caption which says "Not a dream! Not a hoax!" is a pretty clear instruction to read it as if it teally happened.
I can ignore the rubrics. But once you have denied the authority of the Canon Keeper you have, I think, denied the existence of Canon. And that might be a perfectly sensible thing to say. "I don't care if a particular sub-set of Spider-Man stories make up the true story of Spider-Man. I just see a lot of comics, cartoons, and movies. Some of them I enjoy. Other's not so much."
You are perfectly free to say "I don't agree with the Pope on this one. In my Head Canon, Thomas is part of the New Testament and there are Four Persons in the Trinity." But not, I think, to say that and still think of yourself as a Roman Catholic.