Here you will add all the security devices in your home. By this we mean the actual sensors and switches--not the control systems. The control system, or alarm panel (Apex, GE Interlogix, etc.) is added on the interface page first. Then you add the actual motion detectors, glass break sensors, and other such devices here. You can have a mix of systems. For whatever alarm panels or interfaces you added, you will be able to add the sensors and devices here.
The Controlled By selection is where you choose what control system or alarm panel is controlling or monitoring this device. You will also have to supply parameters specific to your type of control system, such as the zone number for security sensors.
Some alarm devices may not need any control system since they may have their own RS232 or Ethernet port. These devices do not require an interface--you can add them directly here. You could also add generic relay-type devices here that use an interface module like the gc100. The gc100 has both relays and inputs, so you can use it to monitor sensors and activate devices.