Currently, the idx is just the index in the array. A future goal of idx is to be a persistent value between GetDeviceList() requests. (i.e. you get the device list, there are four devices, you open the device at index 2 and then on the next GetDeviceList request, the devices could be 0, 1, and 3). The same can be accomplished at the moment on the developer side by using the serial number.