I wrote up a piece on "What is canon" a while back, and then Marc started sending it out to people in response to questions, so it's at least partially correct:
I'm not speaking for Marc here; I'm drawing from personal experience and observation to infer basic facts to assist. If I do speak for Marc online, I'll preface the post with "Marc is asking..." or "Marc wants...", or something similar. Rob Eaglestone also does this.
That doesn't mean Rob or I have any special status. If I was writing material for the Spinward Marches, I'd probably have Hans review it before giving it to Marc for review, because I trust his knowledge of things I've forgotten.
So, bearing all of that in mind, here's some simple facts about canon:
1. It is, at any given moment, what Marc decides.
2. Marc reserves the right to change his mind.
3. The GURPS Traveller universe (GTU?) and the Official Traveller Universe overlap; exactly where they overlap is questionable, depending on who's asking the questions.
4. The precedent of "mostly recently published source" applies, unless that newer source is wrong about something; it happens.
5. Never, ever assume that current licensees are not canon; rather, current licensees are always canon, unless they print something wrong.
6. Marc decides who is wrong.
7. There's no order to the six rules above.
That's pretty much it. On an average day of research for Marc, I'll start with MGT (if possible), then GT (including IW), then TNE (because I have to, not because I want to), then MT, then CT materials. Within those materials, the primary publisher is the first source, but any licensee could be a secondary source. There are probably *golden* sources, but nothing is perfect.
Oh, and yes, 1248 goes in there somewhere. I wouldn't consider it a primary source, but definitely a secondary source.
Heck, there's even a couple of Judges Guild items Marc mentions from time to time. Admittedly, I haven't dug much out of the JG and Paranoia Press materials for Marc, but I've got them to refer to.
And even HIWG docs and fanzines COULD be a source if it's an idea that Marc likes. For example, I pay very close attention to HIWG docs written by Clay Bush on subjects. And I've got the TML and Xboat archives setup as searchable because they have clues to things I don't remember.
I cannot think of a single source that would be outright rejected with no consideration at all. My own timeline isn't a source; it's a list of sources.
So, the moral is, don't throw rocks at other Traveller settings. We're all in the same tent, even if we imagine we're not.
Some things Marc has repeatedly said are NOT canon:
1. Anything under 100 tons jumping. 101-ton jump torpedoes might work, but 99-ton jump torpedoes simply don't.
2. DGP's Aslan art has the hands/paws wrong. I've repeatedly had this explained to me, but I still don't picture it properly.