Manual:$wgRightsUrl

Details
Set this to specify an external URL containing details about the content license used on your wiki. It's set at install time by the installer, either from its own predefined list or from the data returned from the Creative Commons license chooser.

If $wgRightsPage is set then this setting is ignored.

In MediaWiki 1.8.2, it appears that $wgRightsUrl governs portions of the Creative Commons metadata generated within function wfCreativeCommonsRdf in file Metadata.php regardless of whether $wgRightsPage is set or not.