What Does Unique Device Identifier (UDID) Mean?
A unique device identifier (UDID) is a unique serial number assigned to each Apple manufactured device. UDID enables the identification, tracking and recording of Apple’s suite of iDevices.
A UDID cannot be allocated to more than one Apple device.
Techopedia Explains Unique Device Identifier (UDID)
UDID is 40-character alphanumeric string of code for each iOS powered device. This includes iPhone, iPad and iPod devices. UDID has many applications, the most important being the ability to identify each device and segregate users according to their demographics, activity within the App Store, etc. Moreover, some applications depend on UDID to operate, such as Apple’s native device syncing feature, which uses UDID to store and manage synced devices. UDIDs are also retrieved by developers for building apps that use user-related data, but this use is limited due to security and privacy concerns.