https://arstechnica.com/gadgets/2018/05/ios-11-4-debuts-today-with-airplay-2-multi-room-audio-support-for-homepod/

Apple's iOS 11.4 rolls out today, and it brings a big update to HomePods: AirPlay 2 with multi-room audio. The update allows the HomePod and Apple TV (the two Apple devices that currently support AirPlay 2), as well as forthcoming third-party AirPlay 2 devices, to play the same audio throughout your home or only in select areas of your home.

All AirPlay 2 devices can be controlled using an iOS device, HomePod, Apple TV, or by asking Siri. Users can say, "Hey Siri, play classical in the living room," and the appropriate audio will play through the AirPlay 2-compatible speakers, but only in the living room. Essentially, the new setup makes it easier for users to play in-sync audio through all (or only some) of the speakers in their home.