Source Canon is the content that a game is based upon, such as the history, culture, groups/factions, and overall feel. Shadow Accord uses the Classic World of Darkness setting, specifically the Dark Ages line of books.
The World of Darkness contains a number of iterations, editions, revisions, and revivals for various product lines. Furthermore, there are sometimes other publishing companies that at one point or another developed content (Black Dog, Onxy Path, Hexagonal, etc). It can be supremely difficult to sort through all the various possible canon sources, but generally period (Medieval) and regionally (Northern Europe) appropriate content takes precedent over others.
It should be noted that while the World of Darkness is the Source Canon for story and characters, this does not always carry over in to the realm of the Game Rules. The Game Rules of Shadow Accord are developed and maintained for streamlined Roleplay interactions in a live environment, and while attempts are made to keep effects and core dynamics as close as possible to the Source Canon it isn't always feasible.