This is an archived snapshot of W3C's public bugzilla bug tracker, decommissioned in April 2019. Please see the home page for more details.
There's no prose explaining what to do when it is set. There clearly has to be something done or two TextTracks will end up sharing the same VTTRegionList and modifications made on one TextTrack will appear the other TextTrack.
Thanks Kyle, fixed in https://github.com/w3c/webvtt/pull/30