This is a modified Chromecast.xml profile for Plex that is specific to the Chromecast Ultra. This allows you to set up custom 'optimise' targets.
Information updated from: https://en.wikipedia.org/wiki/Comparison_of_video_container_formats https://developers.google.com/cast/docs/media
If any of the information in this profile seems wrong, please open a PR or issue to fix it. I had to make some guesses in places.