The UDID is the abbreviation for the name "Unique Device Identifier". This is an identification number, that only Apple devices use. It is clear, cannot be changed and therefore remains constant, regardless of whether or not the terminal ownership changes.
If you hire an agency such as ours to develop an app, then we can create a so-called "Provisioning profiles. The profile allows the application to be released for your UDID only. This has the advantage that you can install your fully developed app directly to your smartphone or tablet after the deposit of the profile, without having to download it from the App Store. So you have the opportunity, even before the publication of the notice in the App Store to check that you are satisfied with the current state of the application.
The UDID is a constant for each mobile terminal from Apple. This identification number is therefore also well hidden, so that it is not easy to read the UDID of your smartphone or tablet. Currently the number can only be determined using iTunes. Therefore, the action is always as follows - regardless of whether you would like to find out the UDID from iPhone or iPad:
The UDID must not be confused with the UUID.
Since iOS 6, it is no longer allowed to Use the UDID in Apps, because it's relatively simple to Scout personal usage data. Instead, UDID now remains reserved under the rights management solution from Apple, while in Apps the UUID (Universally Unique Identifier) is used.