Audio/video master station for AX Series connects to up to seven additional master stations and up to eight door or sub-master stations (sold separately) to create a security system with video feed throughout a building 3-1/2' LCD color monitor activates during an incoming call from a video door station (sold separately) to aid visitor identification Communication system can be expanded with the Aiphone AX-16SW (sold separately) to include up to 112 additional sub-master or door stations Door-release button controls an electric strike or magnetic lock (sold separately) to remotely unlock the entrance for a visitor Each master, sub-master, and door station has its own labeled button to enable selectable and direct calling