From Amarok Wiki
Why new UIDs?
Why is a new way to generate a pseudo useful UID needed? There are definitiv more useful ways to do it. For example there is PUID (used by MusicBrainz for example).
Small example:
$ ogginfo 01\ Bleed\ For\ Ancient\ Gods.ogg
...
ANALYSIS=MusicMagic DataAgEAMCNx0b2XUI4rgK3UIGpHgAGCU5IRquqUd2CN0186x6STgvOqDLB+gqKkJoYLgFWAAWOagAE/CMLOAj/DK8J7xYeg6hvvgAKAAc2JmXmUYYDSw9NcsYG+gj6W+LDAkyhhF/MGN1KntoNfssizK4VcqHOFWYG0gAFdQ4P8Nzy89wvi0xDHSdocpWMcLIACgAE= FINGERPRINT=MusicMagic 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 ALBUM=The Avenger ARTIST=Amon Amarth DATE=1999-09-02 DESCRIPTION= GENRE=Melodic Death Metal KDE-ENCODER=kio_audiocd MUSICBRAINZ_ALBUMARTIST=Amon Amarth MUSICBRAINZ_ALBUMARTISTID=5b687684-ad34-4a9f-b425-0e7aa81fbd38 MUSICBRAINZ_ALBUMARTISTSORTNAME=Amon Amarth MUSICBRAINZ_ALBUMID=4307faa0-dc71-4e33-ba17-d6e3b31741f3 MUSICBRAINZ_ALBUMSTATUS=official MUSICBRAINZ_ALBUMTYPE=album MUSICBRAINZ_ARTISTID=5b687684-ad34-4a9f-b425-0e7aa81fbd38 MUSICBRAINZ_NONALBUM=0 MUSICBRAINZ_SORTNAME=Amon Amarth MUSICBRAINZ_TRACKID=94db8ca8-f06b-4680-ac75-e2e41c0ed8b7 MUSICBRAINZ_VARIOUSARTISTS=0 MUSICIP_PUID=d7d52f27-16d8-a5ea-3cff-5266b70c6e83 REPLAYGAIN_ALBUM_GAIN=-9.53 dB REPLAYGAIN_ALBUM_PEAK=1.24638510 REPLAYGAIN_TRACK_GAIN=-9.87 dB REPLAYGAIN_TRACK_PEAK=1.24638510 TITLE=Bleed for Ancient Gods TRACKNUMBER=1 TRACKTOTAL=7
With such kind of informations, direct connections to musicbrainz are possible.
In MP3s there is also something called UNIQUEFILEID (MusicBrainz creates them for example) looking like that:
UNIQUEFILEID = http://musicbrainz.org|0393b472-c3b6-4aa7-ad13-08697e96bb19
iTunes Capabilities
The combination of iTunes with OS X 10.4 produces the capability to seamlessly move files around (including renaming files) without the music being dropped from the library. iTunes + Windows fails to reproduce this. I have not tested this with external drives, networked drives, pre 10.4 versions of OS X, r pre-6.0 versions of iTunes.
Upon inspection, it seems there is not extra meta-information written to the tags. (It would be interesting to see what pre-Spotlight OS X versions did in iTunes with renamed/moved music. To reproduce, uncheck in preferences 'copy all music to library folder,' add some music, and move the music around the hard drive.)