AirPlay 2 is an update to the AirPlay wireless standard that Apple has been using since 2010. The new update adds multi-room audio, HomeKit integration, and a raft of other audio-related improvements. AirPlay 2 debuted almost a year ago at WWDC 2017, and while it was present in early betas of iOS 11.3 and tvOS 11.3, it was later removed.
Also in the iOS 11.4 public beta is the HomePod stereo pairing, but since this requires a yet-to-be-released HomePod update, there's no way to test it.
Also, the release includes support for Apple's new ClassKit framework, allowing developers to add support for the upcoming Schoolwork app to their own apps.
If you want access to the public beta, you need to sign up. You can do that here. Before you go any further, you might want to look at my list of precautions and caveats related to running beta software on your iPhone. While you can easily roll back from running the beta, it's possible for the beta to cause havoc in the meantime.
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