EDITOR'S NOTE:
The above was sourced from transcribed copies of the original prose published in the UK version of
Games' Workshop's house magazine, White Dwarf, issue #161.
This prose was written LONG before The Horus Heresy series of novels was ever planned, much less
possible.
It is most definitely not canon.
It is, however, fun to see what kind of prose inspired what has now come about.

 Lord Damocles wrote:
Which is weird, because if 'everything is canon and nothing is canon', how can something be 'most definitely not canon'?
Almost like it's nonsensical gobbledegook...

 Lord Damocles wrote:
Which is weird, because if 'everything is canon and nothing is canon', how can something be 'most definitely not canon'?
Almost like it's nonsensical gobbledegook...

"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 incomplete and 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 when 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 seek is “Yes and no” or perhaps “Sometimes”. And for me, that’s the end of it.

Now, ask us some specifics, e.g. can Black Templars spit acid, and we can answer that one and many others. But again, note that 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, and 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. It 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 nuclear war; that nails it for me."
— Marc Gascoigne, former head of Black Library.

Charax wrote:
I'm inclined to disregard both "Some anonymous editor who copy-pasted a bunch of WD articles and chucked the PDF on some random-ass website" and "playtester's homebrew lore" as factors on what is or isn't canon.

The original articles? Those were published in WD, by GW, they're canon, in that they fall into the "everything is canon/nothing is true" pseudo-canon that everything published by GW falls into. I can grab a bunch of old articles, stick them in a PDF (metadata says the Emperor and Horus was made using Calibre, that's not even the best tool to perform that function in!) and add a little note saying they definitely aren't canon, does that make me the arbiter of canonicity now?

Hell, I've even playtested some GW products! even better, that means I can declare my homebrew canon!

Marc Gascoigne was pretty clear about this on the old BL forums (I remember this post, I was there):

"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 incomplete and 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 when 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 seek is “Yes and no” or perhaps “Sometimes”. And for me, that’s the end of it.

Now, ask us some specifics, e.g. can Black Templars spit acid, and we can answer that one and many others. But again, note that 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, and 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. It 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 nuclear war; that nails it for me."
— Marc Gascoigne, former head of Black Library.

If it has the 40k logo on it, it exists in the 40k universe (I am going to make the reasonable assumption this means legally has the 40k logo on it, not just someone deciding to paste it in)
White Dwarf? Has the 40k logo on it
Imperial Armour? Has the 40k logo on it
Random-ass PDF? No 40k logo
Playtester's facebook post about his homebrew? No 40k logo

If GW publish it, it's canon. but GW have to publish it.
Not someone who compiles GW articles and then editorialises about it
Not someone who did a job unrelated to defining the background of the 40k universe and then drops his homebrew on facebook.

Charax wrote:
"Appear in WD and the SM rulebook"

What do both of those things have on them?

jareddm wrote:

 Lord Damocles wrote:
Which is weird, because if 'everything is canon and nothing is canon', how can something be 'most definitely not canon'?
Almost like it's nonsensical gobbledegook...

That statement has always had caveats, in that there is a small amount of stuff that is explicitly non-canon. 15 years ago, they announced the Heretic Tomes. Three novels at the time, Ian Watson's Space Marine, Pawns of Chaos, and Farseer, were stated to "no longer accurately reflect the fictional universes of Warhammer and Warhammer 40,000," and were stamped to indicate this status.

Since then, authors have come out and said that certain sources are not allowed to be referenced in their writing and likely never will be referenced again, but that they're not allowed to say specific titles. Effectively making these non-canon even if we don't know it.

jareddm wrote:
My gut feeling is when that editor is saying, "It is definitely not canon" what they mean is that it is not true. The story still exists, and someone in-universe believes it, but it's not what actually happened. Anytime the Horus Heresy gets involved, things tend to get muddier because the Horus Heresy novel series, unlike almost every other 40k source, gets stated to be the actual events that happened. Not a retelling, not rediscovered lore, the actual events.

That said, I have another source I'd like to test your theory. Fanatic Magazine #43, On Angel's Wings, written by Ben Dell. It is the only source, as far as I could find, that ever shows a Cherub made from an actual child, rather than vat-born. The article itself states right at the start, "this is Ben's take and house rules on the Cherubim and shouldn't be considered as official." In such a case, if someone were to ask, are there Cherubs made from real babies? Yes or no?