From MusicBrainz Wiki

A URL in MusicBrainz is a specific entity representing a regular internet Uniform Resource Locator. A MusicBrainz URL entity can be edited to change the underlying internet URL it points to; and can be linked to Artists, Releases, Tracks and Labels.

You can take a look at the URL relationship definition trees on the MusicBrainz server. You must be a Relationship Editor to change these definitions.

URL Artist—URL Release—URL Label—URL Track—URL

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