The Relay boxes are used for switching electrical circuits. They can handle quite high currents at either DC or AC mains voltages. Max current is 20 Amps.
They can be remotely switched by text message to either directly turn the relay on or off, or a timed setting can be setup to enable switching at preset times on a daily basis.
The boxes may be used for switching lights on while you are away from the boat. More usefully they can switch small battery chargers for batteries which are not charged by the normal main charging system. The battery voltages can be measured using a battery box so the charger can be switched to keep the battery nicely charged during long periods you may be away from your boat.
Up to 6 relays can be on the system as with other peripheral boxes. If connected to the system using RF then they need a local power supply of 12v DC. 24v relay boxes can be supplied if that is the preferred operating voltage.