Picking and choosing what's canon defeats the point of canon. I don't interpret the law according to how I feel, do I?
Lets not get confused here, canon and interpretation are not the same thing.
Black Library publications are essentially fanfiction with considerably more weight and influence behind them than the average member of DeviantART.
Naw, actually, you're dead wrong, dude. You're seeking "canon" in the sense of something that doesn't exist in 40K - because it doesn't exist, you're going to keep being wrong when you make those claims about what is and isn't canon. You're going to continue missing the point until you adjust that outlook - which isn't easy; I had to have the IP department and the IP manager (among other GW brass) directly explain it to me several times. Read the article linked earlier, or any of my posts, for the actual explanation of how 40K's creators deal with canon.
You're free to keep ignoring them, of course. I'm not sure why you still are - it's starting to look deliberately obtuse, in all honesty. Black Library is as canonical as Forge World and the games studio. That's just how it is, whether you think it rocks or sucks. That's how GW itself views it.
You don't have to see it the same, but that's your preference, not how it is.
You see "picking and choosing canon as defeating the point of canon." You're absolutely right, which is why 40K doesn't have canon in those terms. That's the point of the setting. It's not about canon and a metaplot. It's a sandbox. It's not bound to the same laws of canon as most other settings. Trying to apply them will always make the applier wrong.
If the explanation in the article and my own posts are still not working for you, here's a quote from Marc Gascoigne:
"The Background exists as a context for the games that people play. Despite the occasional event, the background was never intended as an ongoing narrative that would be constantly updated. The back story presents questions, enigmas, problems, and conflicts. Gamers explore and solve these issues by playing games and developing armies. In short, the background provides the beginning, but the players provide the end.
What is Cypher up to? Well, he's up to whatever you need him to be up to for your games and campaigns. What does the cult mechanicus have to do with the dragon? Whatever you want that relationship to be.
The background should be like Schrödinger's Cat - Nothing is defined until the players look into the box by playing games and determining the outcome for themselves. Backgrounds should be full of possibilities to be exploited and expanded by players, not answers that limit the potential of the game and its setting.
"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."