Each Track in MusicBrainz has a unique TrackID, 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/track/2c2548c1-276a-44e1-a2ea-84b4abe31be9 is a unique identifier for the track One More Cup of Coffee (Valley Below).
2c2548c1-276a-44e1-a2ea-84b4abe31be9 is a relative URI for this track.