" Often folks ask if Black Library books are ‘canon’. With Warhammer and Warhammer 40,000, the notion of canon is a fallacy. There are certainly established facts – the current Emperor is Karl-Franz, the Blood Angels have red armour, Commissar Yarrick defended Hades Hive during the Second Armageddon War. However, to suggest that anything else is non-canon is a disservice to the players and authors who participate in this world. To suggest that Black Library novels are somehow of lesser relevance to the background is to imply that every player who has created a unique Space Marine chapter or invented their own Elector Count is somehow wrong. Nothing could be further from the truth. Warhammer and Warhammer 40,000 exist as tens of thousands of overlapping realities in the imaginations of games developers, writers, readers and gamers. None of those interpretations is wrong.
…
Whether a particular author’s take on the world matches up with an individual gamer’s or readers is another matter. The fact that each of us is allowed to take possession of that world and envisage it to our own ideal means that it is inevitable our vision will sometimes clash with the vision of others. Such conflict does not render either vision obsolete.
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In this regard it is the job of authors and games developers to illuminate and inspire, not to dictate. Perhaps you disagree with the portrayal of a certain faction, or a facet of their society doesn’t make sense in your version of the world. You may not like the answers presented, but in asking the question you can come up with a solution that matches your vision. As long as certain central themes and principles remain, you can pick and choose which parts you like and dislike."
- Gav Thorpe

"It all stems from the assumption that there's a binding contract between author and reader to adhere to some nonexistent subjective construct or 'true' representation of the setting. There is no such contract, and no such objective truth."
- Andy Hoare

"There is no canon. There's a variety of sources, many of which conflict, but every single one is a lens through which we can see the 40K setting."
- Aaron Dembski-Bowden

"Keep in mind Warhammer and Warhammer 40,000 are worlds where half truths, lies, propaganda, politics, legends and myths exist. The absolute truth which is implied when you talk about “canonical background” will never be known because of this. Everything we know about these worlds is from the viewpoints of people in them which are as a result incomplete and even sometimes incorrect. The truth is mutable, debatable and lost as the victors write the history…

Here’s our standard line: Yes it’s all official, but remember that we’re reporting back from a time where stories aren’t always true, or at least 100% accurate. if it has the 40K logo on it, it exists in the 40K universe. Or it was a legend that may well have happened. Or a rumour that may or may not have any truth behind it.

Let’s put it another way: anything with a 40K logo on it is as official as any Codex… and at least as crammed full of rumours, distorted legends and half-truths.

I think the real problem for me, and I speak for no other, is that the topic as a “big question” doesn’t matter. It’s all as true as everything else, and all just as false/half-remembered/sort-of-true. The answer you are seeking is “Yes and no” or perhaps “Sometimes”. And for me, that’s the end of it.

Now, ask us some specifics, eg can Black Templars spit acid and we can answer that one, and many others. But again note thet answer may well be “sometimes” or “it varies” or “depends”.

But is it all true? Yes and no. Even though some of it is plainly contradictory? Yes and no. Do we deliberately contradict, retell with differences? Yes we do. Is the newer the stuff the truer it is? Yes and no. In some cases is it true that the older stuff is the truest? Yes and no. Maybe and sometimes. Depends and it varies.

It’s a decaying universe without GPS and galaxy-wide communication, where precious facts are clung to long after they have been changed out of all recognition. Read A Canticle for Liebowitz by Walter M Miller, about monks toiling to hold onto facts in the aftermath of a nucelar war; that nails it for me.
Sorry, too much splurge here. Not meant to sound stroppy.

To attempt answer the initial question: What is GW’s definition of canon? Perhaps we don’t have one. Sometimes and maybe. Or perhaps we do and I’m not telling you. "
-Marc Gascoigne