We already had "${DEVICE}" which uses the interface name.
In times of predictable interface naming, that works well.
It allows the user to generate IDs per device which don't
change when the hardware is replaced.

"${MAC}" is similar, except that is uses the permanent MAC
address of the device. The substitution results in the empty
word, if the device has no permanent MAC address (like software
devices).

The per-device substitutions "${DEVICE}" and "${MAC}" are especially
interesting with "connection.multi-connect=multiple".