Unique Device ID or UDID is a number that all Apple devices use. It is used when they communicate with the Apple’s servers in order to restore a firmware or to use iCloud. It is also used for Find My Phone feature. Keep in mind that this system isn’t used by Android devices.
Unique Device ID originated in 2007, obviously in the United States and it was requested and approved by the FDA, as a part of the Food and Drug Administration Amendments Act of 2007.
This number contains the serial number of a device, so it is important for targeting the specific devices or better said the specific version. As such, it is essential to understand and use by all who are working with Apple devices or in the marketing industry.
Target marketing which is based on device versions or the device type wouldn’t be possible without UDID. This means that marketers use it in order to serve the suitable ads which are developed for a specific Apple device.