In addition to the serial number, each iOS device has a special 40-character long identification code called UDID. UDID stands for Unique Device Identifier. It looks like this: 3c6f0cc904d137be2e1730435f5664094b831186.
Developers need to add the UDID to the beta application to be able to install it on your iOS device. Just like people need their ID to board a plane, iOS devices need to have their UDID registered to join the test.