Canon is the whole of facts that are considered true either within a certain context or by an organization. Examples of Monster High canon are that Cleo de Nile is afraid of the dark, that the 2011 TV special is called "Fright On!", that the Monster High book series contains four books and three chapters 13 and one diary, and that Toralei Stripe is voiced in English by America Young. Ever After High, as the sister franchise of Monster High, is also considered canon for Monster High.
The Monster High Wiki only accepts contributions that relate to canon (with one exception). It used to accept non-canon contributions in the form of Original Characters until April 16, 2011, at which point the Monster High Fandom Wiki was founded. If you wish to tell the world about your OC, please post information on them on the Monster High Fandom Wiki.
What is Monster High canon?
As per this article's intro, Monster High canon is every bit of information regarding Monster High that is true—"true" being defined on the Monster High Wiki as "by Mattel or with approval of Mattel".
The one gap the above definition doesn't answer is how the wiki deals with fan material being acknowledged by Mattel. Such cases include the December 2011 Create-A-Monster profiles, several reblogs on Tumblr, and the handful of custom dolls produced for the winners of Monster High contests in several countries. Though the wiki may report on this fan material, it is not considered canon because this material falls outside of Mattel's (lasting) control and thus out of the control of the sole authority of the Monster High franchise.
The Monster High Wiki considers there to be no hierarchy in Monster High canon. That means that if, for instance, the facts presented in the doll logs clash with the facts presented in the Ghoulfriends book series, both sets of facts are considered canon and worth reporting, but only for their respective continuities. The only sections of information on the wiki that may be written in a pan-continuity manner are characters' Character sections and locations' History sections, which detail the average of canon on those topics.
The way a name is spelled is also a matter of canon, and not all names are always spelled the same. For instance, Hoodude has been canonically spelled "Hoodude", "Hoo-dude", and "HooDude", and Van Hellscream canonically "Van Hellscream" and "Van Helscream". In such cases, the trademark is considered the correct spelling. If there is no trademark, then the spelling most used is considered correct. And if that doesn't solve the matter, then the spelling used in the source closest to the relevant fiction (ie, credits are more worthwhile than the Gory Gazette) is considered correct. A special case are the webisodes, of which the true name is considered the one written out in the webisode itself. The one exception is "Night of a Thousand Dots", which promotional name is much less ambiguous than its true name "Dot Dead Gorgeous".
There is one exception to the No Fanon-rule and that is the naming of some of the doll assortments. A small number of assortments have no name at all attached to them. Often, the Monster High fandom makes up for this by inventing a name themselves. Examples of this are 'Killer Style' and 'School's Out'. The Monster High Wiki has opted to use fanmade assortment names if there are no official ones available. It is, however, at any opportunity repeated that the used name is fanon: on most pages by the quotation marks around the name and on the doll assortment's own page within the infobox and within the Name section.
It is common on wiki's to have items in the Notes or Trivia sections about how a character or event in another franchise bears a similarity to the article's topic. The Monster High Wiki, trying to stick to canon, does not join in this. Unless there is reason to believe the similarity is intentional, similarity-trivia is not to be added to the Notes sections.
Mattel and fans
A very important detail about how the Monster High Wiki documents information is that the Monster High Wiki is not interested in scoops. What the Monster High Wiki is interested in is verifiable, accurate, and fairly obtained information.
There are two problems that may arise when aiming for a scoop: damage to Mattel's marketing plan and damage to fan enjoyment.
Mid-2012, information about future doll releases - including photos - spilled onto the internet due to a misunderstanding between various sources. None of this information was supposed to be undisclosed yet and Mattel immediately took action against all sites that had the photos uploaded. The Monster High Wiki was also targeted due to eager fans uploading the photos several times. This was an unpleasant incident the wiki rather sees unrepeated. If information about the Monster High franchise is not obtained from an official source (this includes all of Mattel's public websites and channels, plus store websites and trademark databases), it is principally not fit to be uploaded. If you can't track the source, ask an admin if they think the information or image okay to be added to the wiki.
If time shows that Mattel doesn't take action to the information being shared online, then of course it automatically becomes suitable for addition. By then it's not a scoop anymore, but the wiki is assured information that is rightfully obtained and accurate.
Another rule of 'avoiding' scoops is that the wiki does not document on webisodes that haven't been officially released in English yet. The Monster High Wiki assumes its readers to speak English and does not assume them to speak other languages too. As such, reporting on a webisode that is released in Spanish/Italian/French/etc. before it is released in English would be akin to posting spoilers, which is not the Monster High Wiki's aim. Also, regarding the matter of accurate information being favored over scoops, chances are that information that can be gathered before a webisode is released in English is inaccurate for the English version, such as the title and the webisode's place in the volume's order. As such, the wiki does not report on non-English and non-official releases of webisodes.