Crossover and/or fusion?

[graculus]

So, in the interests of furthering my chances of writing stuff for trope_bingo, would anyone like to have a go at explaining to me the what makes something a crossover fic rather than a fusion fic, preferably citing examples of fandoms I actually know something about (i.e. not Teen Wolf or Supernatural)?

Because I have managed to get a card with both and can't really see what the difference is from the definitions linked from the community...

Comments

[graculus.livejournal.com]

So, it kind of sounds as though it's about dissonance - in a crossover, there's a clear clash of worlds and in a fusion there isn't? That makes much more sense than the definitions I'd seen, which seemed to be saying that X is Y but X can also be Z.