Apple has added AirPlay 2 support into iTunes as part of this week’s macOS High Sierra 10.13.6 update.
While iTunes has long supported multi-room audio using the standard AirPlay protocol (and AirTunes before that), the addition of AirPlay 2 support for iTunes will allow for faster performance when switching between and streaming to multiple speakers, better audio synchronization, and, perhaps most significantly, multi-device control, allowing you to begin streaming audio from your Mac or PC and then subsequently control it from an iPhone, iPad, or even an Apple Watch.
AirPlay 2 support is also naturally included in macOS Mojave, currently in public beta, but will be a welcome surprise for those users who are still on High Sierra.