What is Canon
in Star Wars?
The word “canon” has several meanings, but in the context of religion it means a “collection or list of sacred books accepted as genuine.”
In the context of fandom, the word has been jocularly appropriated to refer to story elements, designs, characters, etc, which are accepted as “real” in the context of the story or the fantasy universe. "Gospel" is a synonym for canon. And like any such definition, it all depends on who's doing the accepting. Especially since we're dealing with fiction and corporate entertainment here.
You might, for example, argue that the Harry Potter books by J.K. Rowling are all canon, since the creator of that universe wrote them, but that a Harry Potter sequel written for fun by a fan is definitely not. Or you might argue that novelizations of the Star Wars films are canon, but that the overall series of Star Wars universe novels are not.
But these are all open to debate and are thus thoroughly contentious. Was the Star Trek animated TV show canon? Is the Star Wars “Extended Universe” material no longer canon even though it was once was? Is the extra stuff from the Star Wars novels canon? Or just the stuff derived from the film scripts? Who determines canonicity? Authors? Franchise owners like Disney or Paramount? Fans? And so on.
Canon for model makers
We have a related concept of canonicity when it comes to making models of fantasy vehicles, places, props, etc. It's not like making a model of a real car or airplane. Those things are genuine objects; documented and researchable. Fantasy stuff obviously ain't real. It's by definition made up!
Degrees of Star Wars Canonicity
Here's a proposed list of different levels of Star Wars canon for the first three Original Trilogy films (1977-1980). I'm mostly thinking in terms of visual design of models, props, costumes, sets, and so on, though you could apply these rough categories to narrative issues as well.
The three OT films as they were first released. Only whatever is visible on the screen counts.
As above, plus any behind the scenes photos, deleted sequences, documentary photos and films shot at the time of movie production.
As above, plus production blueprints and drawings which do not conflict with above.
As above, plus photos of original models/props/costumes as they exist today.
optionally accounting for any changes since production.
As above, plus the CGI-modified Special Editions.
optionally only those SE changes which don't conflict with above.
As above, plus designs and illustrations in Lucasfilm-sanctioned publications (books, "technical manuals", multi-coloured "blueprints", etc) and commercial products (toys, models, etc) created as ancillary material after the movies were produced.
As above, plus changes introduced in Lucasfilm animations (The Clone Wars, Rebels, etc)
As above, plus changes introduced in Disney-era/post 2012 films (TFA, TLJ, R1, Solo)
As above, plus popular fan designs.
As above, plus the Holiday Special.
Like anything in fiction, there's no right or wrong. It's all about what interests you and where you choose to focus those interests. In my case, I generally try to build models that fit categories 1 through 4 or 5.
I basically mention all this because I think there tends to be an ideological breakpoint at roughly categories 5 and 6, and this causes some confusion when people assuming one side of the line argue about stuff on the other side of the line.
Is there even a point?
Of course, it's not even as simple as this proposed structure. Because there's a lot of inconsistency within categories. There were often many different props used for the same vehicle or character or object within the same film, for example. R2-D2 was played by multiple physical droid props. Luke didn't always use the same lightsabre. And so there's a lack of consistency within each film as well.
Naming is another area of nerd contention. Take, for example, the names of the various parts of R2-D2. The "radar eye" or "coin slots" or the "octagon ports" or whatever. The problem is:
the film scripts refer to some things one way.
the novelizations refer to some things another way.
the blueprints occasionally refer to some things.
the original builders in Britain probably casually referred to some bits other ways.
the staff at ILM may have had names for various things.
many parts weren't given names by anyone.
years later various "blueprint" books and other ancillary marketing materials were released, and their authors came up with other names.
hobbyist builders have come up with names for some things.
the prequels came out and the builders of those props called things various ways.
In the end, it's about what works for you and makes you happy!