The radio stack is a feature of the GPS software (Plan-G by TASoftware) I'm using. It can connect to X-Plane through XPFSUIPC, open FMS files, so the flight plan in FMS is displayed on map. Your aircraft through XPFSUIPC can be tracked in real time and you can track yourself and other aircraft on VATSIM, however I find quite a delay (Up to 100 miles) in updating positions. You can enter values direct in the radio stack and, through XPFSUIPC, it updates in most aircraft. It was a very useful tool when I first started with X-Plane.