Unique Device Identifier (UDID)
A unique device identifier is a string of 40 letters and numbers that corresponds to a given Apple device. Each device has a UDID and no two UDIDs are the same, which used to allow apps and third parties to track the behavior of the device owner.
This was a benefit for many companies, such as advertisers who could offer targeted ads based on the behavior and interests of the user. UDIDs were a big part of how apps available for iOS functioned until security concerns forced Apple to phase out apps that requested the data. Now, Apple uses this information for cloud computing, iTunes, Findmyphone, and similar company projects but do not allow this information to be given to third parties.
UDIDs are associated with the motherboard of a device and cannot be changed once they are set unless the motherboard itself is reset. Apple guards UDID data closely and no longer allows third parties to access the information.
A professional mobile app development company may be able to provide more information about UDIDs and other options now available for accessing data.