I look at FF spinoffs as a continuation of a FF world, as an expansion of sorts.
I suppose Crystal Chronicles to be fair is it's own thing. As it too has it's own unique world.
That's a good definition. Unfortunately, in a series like FF where every numbered game is set in its own world it makes things unclear. Spinoffs generally refer to stories following a side character from a core title that have little or no relevance to the main storyline. It can also refer to a series that is spawned from another series though. Meaning sub series are spinoffs. Either way, they aren't mainline.
Stop your ignorance of Final Fantasy XI's awesome story. See it here. http://ffxicutscenes.blogspot.com/