as far as i can gather, the airplay in the music server is there to make airplay compatible output devices appear as external zones, so some airplay speakers can be used as a player for the music server, similarly there is facility to support upnp/dlna players in a similar manner
Stream Music Wirelessly via AirPlay
Time to get the party started! Send music from your AirPlay-enabled devices (iPhone, iPod Touch, iPad and Mac) straight to the Loxone MusicServer. With AirPlay you also have access to an unlimited number of music services from the AppStore.
With services such as AirParrot, AirPlay can also be used on Windows devices.