Each Label in MusicBrainz has a unique Label ID, which is an absolute URI. It has the form
UUID is a Universally Unique Identifier in its 36 character ASCII representation. In situations where the context is clear (such as file tags), just the
UUID part can be used, leaving a relative URI.
http://musicbrainz.org/label/713c4a95-6616-442b-9cf6-14e1ddfd5946 is a unique identifier for the label Blue Note. 713c4a95-6616-442b-9cf6-14e1ddfd5946 is a relative URI for this label.