If you want to stream music to your Cast-enabled speakers, YouTube Music will suggest which one to use. The app would simply list all of the Cast-enabled devices on your network in order. If you're anything like us, you'll have to look through a list of half a dozen speakers, even if there's only one you use on a regular basis.
A new menu with a suggested device highlighted above the standard alphabetical list has been spotted by ChromeUnboxed. The new feature is based on usage, history, and the type of device, according to Paul Pennington.
The feature that is specific to YouTube Music has been rolled out to all users of the app. Unless you have uploaded your own music, you can't cast music from the app to an external device.