HG must be connected to a X10 controller of some kind (CM15, CM19, etc). That controller sends/receives X10 commands. When you set up HG to have a module, HG doesn't really care what type it is other than knowing that it is there. If you set a light, switch, etc, the only difference is the icon that shows up in the interface. If you don't like the icon, you can actually change it.
In any case, HG will send a PLC/RF command when you click ON/OFF in the interface through the connected controller and it deals with it from there. If the controller sees a PLC/RF command for ON/OFF, it will be seen by HG and it will switch the light/switch to the ON/OFF icon and change the status.
IOW, you can't have more than one device on the same HU code because HG doesn't see it that way. I don't have my motion sensors on the same HU code as my modules and I use HG to interpret the action. If you set it up like X10 initially envisioned, you don't need HG involved at all (with the motion sensor). Simply ignore it. If you want to use the motion sensor in another way, change it to a different HU code and utilize some kind of script to make it do something smarter. For an example, see the Advanced Smart Lights code.