UDID

UDID is an acronym for Unique Device Identifier. The UDID is a feature of Apple's devices running iOS, tvOS, watchOS, and macOS. It is a unique identifier that is calculated from different hardware values, such as the ECID. It is sent to Apple servers when a user tries to activate the device during Setup. This ID is also used to detect the phone or to communicate with it while restoring the IPSW firmware. From Wikipedia