YouTube Music Key is rolling out today, and only All Access subscribers are getting it. The app presents offline playback from YouTube at 360p or 720p, ad-free music videos and background playback. If you subscribed and have the app on your mobile, you can use it on the web as well.
The rollout is limited to Play Music All Access subscribers, even those who didn’t subscribe to the app specifically. An expanded rollout to those who subscribed for YouTube Music Key could follow in a few days.