The Ocean Reef Alpha Pro X-Divers Underwater Unit only is the communications headset that attaches directly to your full face mask. The Ocean Reef Alpha Pro X-Divers Underwater Unit is specifically designed to be used with the Alpha Pro X-Divers Surface Unit only. The Ocean Reef Alpha Pro X-Divers Underwater Unit is designed for professional or commercial divers who require a full duplex, hand free, clear communication system. The range of communication with the Ocean Reef Alpha Pro X-Divers Underwater Unit depends on the length of cable used. The Ocean Reef Alpha Pro X-Divers Underwater Unit has all the same features as the other Ocean Reef Communication units except this also has a recall button.