The Manifest.plist is another file in the root of the iTunes backup folder.
Like the other plists, this one lists the device name, serial number, and Unique Device ID. Some of the additional values included are identifying if the backup is encrypted, syncing to iCloud, if there is a passcode set on the device (different from backup encryption), WiFi sync capabilities, and any accessibility features enabled.
A note about the com.apple.mobile.wireless_lockdown. This is not indicating whether or not the device has WiFi enabled. This key shows whether iTunes can be synced over WiFi.
Also note (unrelated to the Manifest.plist), if you want to prevent the phone from automatically syncing when it is plugged into your computer, uncheck the box “Automatically sync when this iPhone is connected.”