With new, a new random identifier will be generated.
With machine-id, the identifier of the current machine will be printed. See machine-id(5).
With boot-id, the identifier of the current boot will be printed.
Both machine-id and boot-id may be combined with the --app-specific=app-id switch to generate application-specific IDs. See sd_id128_get_machine(3) for the discussion when this is useful.
With invocation-id, the identifier of the current service invocation will be printed. This is available in systemd services. See systemd.exec(5).
With show, well-known UUIDs are printed. When no arguments are specified, all known UUIDs are shown. When arguments are specified, they must be the names or values of one or more known UUIDs, which are then printed.
The following options are understood:
-a app-id, --app-specific=app-id