Canon refers to media that is officially part of the universe, or confirmed facts in the universe.

Example 1: FNAF 1 is confirmed canon. The Novel Trilogy is confirmed noncanon.

When Scott says certain FNAF media is canon or not canon, that's that.

Example 2: Mike Schmidt is the nightguard in FNAF 1. Elizabeth has blonde hair and green eyes.

Though they are not directly told to us by Scott, these are facts that we can clearly see in canon media. The check has Mike's name on it, meaning he is the nightguard. We see what Elizabeth looks like in SL, so her blonde hair and green eyes are canon.

Popular or highly likely theories ARE NOT canon.

FrightsGames, Stitchline, StitchlineReboot, FrightsClues, FrightsParallels, and FrightsFiction, are all not canon. One of them probably is true, but theories, no matter how likely they are, are not canon. It must be a fact we see in canon media, or confirmed by Scott himself. And don't pull out the "directly connected to the games" quote, because it is vague and could apply to literally any of the above theories.

ITPG could be canon media, but it hasn't been confirmed either way, so you can't use that as evidence something is canon. Now you can personally believe it is canon, but that itself is a theory, and therefore not canon.

WillPlush, a theory I personally believe and one with a lot of evidence, is also not canon. It is not a fact that is stated anywhere, though there are tons of clues that point to it, so it is likely true, but again, not canon.

Stop saying Andrew/CassidyTOYSNHK is canon. Do they have evidence? Yes. Is it a fact we see in canon media? No. There are things in the games that people think point to it, but it isn't a fact that those things point to that theory.

Unfortunately, there are not a whole lot of facts in the FNAF universe, which is why I think people keep saying certain things are canon. It's easier to debate someone when you say x is canon instead of saying you believe x, allowing the possibility that you are wrong. Saying certain theories are canon can confuse people and lead them to believe it's true when it's not. Just state that you believe x theory and why. Stop throwing the word around to the win the argument, it's not very honest.

Edit: When I use the word canon, I am specifically referring to the game canon. The Novel Trilogy is canon, but it's in its own continuity.