Can you tell me more about Unity's Human Interface Devices?

Questions & Answers
1

-Game Specific Interface
-Physical Device

2

Are you trying to connect anything specific to a Unity application?
Generally, you are not really limited in any way. Any standard USB input device that works under Windows should be fine. For anything else (Kinect, custom devices, trackers), you can simply communicate with these devices via plugins.
Maybe check out this tool as well:
http://forum.unity3d.com/threads/97995-MiddleVR-a-Virtual-Reality-plugin-!