Each DHCP client and server has a DHCP unique identifier (DUID). A DHCP server uses DUIDs to identify clients for the selection of configuration parameters. DHCP clients use DUIDs to identify a server in messages where a server needs to be identified. Clients and servers treat DUIDs as opaque values and use it compare for equality.
The DUID is not required in all the DHCP messages. It has to be unique across all DHCP clients and servers. It has to be stable for any specific client or server - for example, a device's DUID should not change as a result of a change in the VLAN‘s port.