Backup and Sync

This is exactly the issue that has got me fairly baffled.  I'm over on the PC world but same gig unless someone can enlighten me.  There seems to be no clear conceptualisation of when you would backup vs sync as they're sort of nested in each other and then this redundant 3rd copy you mention which as someone who works with tons of different versioned docs seems a recipe for big trouble.

Once again I keep feeling like I'm missing a point here .. that there's got to be some sort of nice concise user documentation explaining all of this and it really should not be that complicated .. should it??