If it's the one I'm thinking of in Settings/Privacy, it pulls whatever apps and settings were uploaded to your Google account during the last sync. Not sure when it syncs though, the last few times it's auto-restored on mine it pulled down wallpapers and apps I haven't used in quite a while. It gets kind of annoying since I use Titanium, so I always turn off the auto backup/restore. I'm fairly certain it only restores after a factory reset or if you manually wipe /system and /data.
My phone doesn't have MIUI yet, so if you're referring to something specific to the ROM then I'm not sure.