The "Enable sharing of book content" checkbox? I've got it unchecked.
It says that checking it will disable sharing. I must have been reading out of date information however on how ASIN numbers are used to sync.
I've not got either option to work.
I use sideloading to the SD Card on the Android device, copy to My Documents on the kindle desktop app, and direct mobi download from content server on the iOS device.
Couldn't get it to work. I'm not here for support though, I can do more reading if the code is useless.