I feel kinda silly. I’m about to say something that can easily be said in a tweet, but unfortunately, there’s still confusion. So here we go.
The definition of canon where it relates to fanfiction:
Canon = Fact.
Everything that happened in the books is fact and therefore canon.
Change a single event or character trait or location from the fact laid out in the books, and it’s not canon. You’re in alternate universe (AU) territory.
That’s really all you need to know, but I’ll talk a bit more so I don’t have to come back.
Confusion about canon comes from the fact that all those little bits that make up canon as a whole can be broken down.
AH Edward/Bella is a canon pairing – but it is not canon, ‘cause, you know, Edward’s human. Edward was never human during Bella’s lifetime. Tell all your AH het writing friends.
Your vampires are venomous, don’t sleep or bleed. Great! We love this. But if Bella gets changed by James in the ballet studio, it’s not canon.
Post Breaking Dawn, Edward and Jacob get together. Great! We love this! But if Edward bites Jake in the heat of passion and Jake walks away from it, it’s not canon.
You need all three of these to have a canon fic. If anything is missing, you’re in AU territory.
Then How Can Slash Ever Be Canon?
You’ve got a whole timeline that stretches from the Roman era (probably before, I’m writing off the top of my head, and I’m not an Aro expert) through to the 21st century and beyond if you want to do future fic.
What was Carlisle up to while he lived with the Volturi? Never is it said that he was a pure and chaste virgin before he married Esme.
Edward and Emmett’s hunting trips always made my slashy little heart leap in my chest.
Kill Bella and Alice after the curtain closes on Breaking Dawn and Edward and Jasper are free to take comfort in each other.
Weird wolfpack initiations that require submission to the alpha? Hell yes 😉
Just because Garrett ends up with Kate in Breaking Dawn, it doesn’t mean he only went for girls before that.
You can be canon pairing compliant in slash fic as long as canon pairings, within the canon timeline, remain intact.
How Do We Know What Is Canon?
You’ve read the books, right? It’s all there. Some of it is pretty vague, open to interpretation. That’s great. It makes us think.
Some things Steph got wrong. Not big on research, it appears. Can create headaches for fanfic authors, I generally say go with book canon, or go AU.
Steph wrote some extras and gave some interviews. Then she wrote a novella. Then a guide was released. If the stuff in there doesn’t contradict canon, go with it, call it canon, no worries. If it does, I’m inclined to go with the original material.
Do I Have To Write Canon?
Hell no! Alternate Universe is fantastic! You have so much freedom, you can twist things, rewrite history, make it so Bella was never born (is it just me that loves this kind of fic?), have your characters do deliciously filthy things they would never do in canon. Just label it AU so we know.
Finally, this isn’t just the VampSlash girls saying this. Canon isn’t really subjective. It’s a cross-fandom acceptance of fact. Once again, I suggest you read giselle-lx’s (one of our VampSlash Contest judges) Canon Fodder pt1. Please, please, please read it. It explains everything very well and you can decide whether you want to write your fic canon or AU.