The Spartan UltraScale+ FPGA contains a physically unclonable function (PUF). The PUF creates a signature or fingerprint of each device that is unique to that device. Its value is not known by AMD or the user enabling usage as a key encryption key (KEK). This KEK is a 256 bit key used to encrypt the users red key allowing its storage in black (encrypted) form. The black key can be stored in eFUSEs or the boot header of the PDI.
The PUF also outputs a user accessible unique ID that is cryptographically isolated from the PUF KEK itself despite using the same entropy source. While unique to each device, it is not considered a secret and does not have the same access protections as the KEK itself.