A USB device that is plugged in identifies itself by its VID/PID combination. A VID is a 16-bit vendor number (Vendor ID). A PID is a 16-bit product number (Product ID). A host uses the VID/PID combination to find the drivers (if any) that are to be used for the USB device. For this to work, the VID/PID combination must be unique, in the sense that each USB device with the same VID/PID will use the same driver. So, whenever you need a specific driver for your USB product, you will need a unique VID/PID for that product. In that sense, the VID/PID combination does not really serve as a truly unique identifier of a single device instance. It is, however, useful to know for medical devices that communicate using USB.