Цитата(ATXmega manual)
The production signature row also contains a device ID that identify each microcontroller device
type, and a serial number that is unique for each manufactured device. The serial number consist
of the production LOT number, wafer number, and wafer coordinates for the device.
Этих регистров очень не хватало в обычных мегах, а тут можно привязывать прошивку к конкретному чипу.