#1 - Faithful Source material
Where a pre-existing book/comic/whatever is directly adapted into a film or TV show, the general assumption is that the original source novel is actually the higher canon and the community will usually rewards answers relating to 'what came first'
#2 - Post-facto Novelisations / Shooting Scripts / Graphic Novelisations / Bonus Materials
This is where the related materials directly relate to the film/show and agree with it almost completely. It is almost universally held that these are at their best when the writer/creator of the original property was also involved in writing of the bonus materials.
#3 - Pre-facto Novelisations / Early Scripts
These are often written before scripts have been finalised and can often disagree with the finished product. They may exclude key events, change names or even included elements that were ultimately deleted from the finished film/TV show. Votes tend to flow very sluggishly if you try to use these sorts of sources.
#4 - Unrelated source material
This is where things get really dicey. It's not unusual for a property be "based on" an earlier work but without actually having any actual similarities in terms of plotting, characterisation or structure. In those cases, using the original material to explain the new property is pretty tenuous at best. For example, Magneto appears in a bunch of comics but that doesn't mean you can point at his abilities in the comics to explain his abilities in the X-Men films.