I think they are globally unique across all LoRa certified devices?
The spec proposes two solutions to commission a note to a network:
Over the air activation: the node has no prior knowledge to which network it will connect. It has a DevEUI bought by the manufacturer to the IEEE space (like a MAC address), and thru exchange with the servers it will get credentials and key
Personalisation at production: here the network you are pre-linked to will supply you with a DevAddr (32 bits), and a set of encryption key. This will be the responsibility of the network authority to ensure all of those are unique on his network
Quote from here