I was led to believe that the add-ons to the main PCM are modules that CAN potentially be added on. So the PCM comprises the core of the screen and buttons and main computing power :-), and the SatNav module or Phone module can then be added on to that. Obviously there is a bit of complication with GPS and phone aerials - I have not actually seen any mention of where they are. Mine will have both so I will hopefully be able to tell in a couple of weeks.