I've been experimenting and messing around with the different sync options to work out whats best for me and I'm confused by a couple of things.

I see that you can select the frequency of background syncing individually for each app that requires it, which is good. There is also a global auto sync on and off checkbox which appears to turn on and off syncing for all apps with a scheduled sync, overriding the individual settings for each app. This much I'm happy with and understand. What I'm confused about is, in settings/accounts & sync there is another checkbox above the auto sync on and off checkbox label "Background Data". You can't turn on auto sync without first checking on "Background Data" but you can have the auto syncing disabled but still have background data enabled. So, what data is getting transfered if you have the auto syncing already switched off?