Canon

Canon is the term used to indicate that a particular rule, piece of lore, etc is from the original Birthright published books. Theoretically this should indicate that the item in question is more reliable than 'house' content, or is truer to the setting. In practice, canon written in one place frequently contradicts canon elsewhere, requires changing due to changes in the system, lacks the necessary depth for game-play, or is just plain wrong for a particular campaign.
Whether something is canon or not tends to matter only between people who rarely play together. Canon then provides them with a common term of reference. This means that if one plays an elf and the other a goblin, then both players know that there will be racial tension issues between their characters without a need for further explanations as to the characters' behaviours. In long standing campaigns the differences are known and are unlikely to be a problem.
In general if something in a campaign conflicts with canon, it is not wrong. Each campaign is right as long as people enjoy themselves. However, in order to avoid conflict or bad feelings between players or prolonged arguments between players and the DM, any change to canon should be explained in advance, much as house rules or even the system to be used would be. Canon is, in effect, the rules of the social setting.