A 'device' is basically a CQC device driver. So Z-Wave is one, RadioRA2 is one, an Elk is one, an Omni is one, etc... In all of these types of devices where there's a controller that CQC talks to and the controller talks to the other things, that's just one device from CQC's point of view, since it only requires one device driver to talk to the controller.
You can make it more physical by saying each of the things that CQC has to connect to, because each connection requires a driver. CQC connects to a Z-Wave or Omni controller, but it doesn't connect directly to the things the Z-Wave or Omni controller controls, so that's just one driver. OTOH, each TV or A/V receiver has its own serial port or ethernet connection, so each of them requires a separate device driver in CQC to talk to it over that specific connection. If you want weather info, that's a connection to a weather server, so it requires a device driver. If you want to manage your media, that's a connection to iTunes or DVD Profiler or My Movies (or in some cases in a more abstract sense a connection to the local drive to expose media files with just embedded tags), so that's a device driver.
Does that make sense?