"Canon" refers to the known FS universe, described by

. There has to be a non-negotiable core to fall back on, and to keep stories coherent.
But there's nothing preventing someone from establishing their own continuity, even though it isn't canon. Several campaigns take place in the Derelict continuity. Several campaigns take place in the Inferno continuity. Many more campaigns are
compatible with certain continuities, even if the campaign designer doesn't explicitly place them in one.
The vast majority of continuities are compatible with canon, even if not many of them are compatible with one another. (I don't think Derelict and Inferno are compatible.) Most campaigns don't contradict canon unless there's a very good reason for them to do so. (Silent Threat: Reborn is one of these.)