Speed signal in models without speed over CAN (A6 C5, A4 B5, A3 8L, A2). A4 B6 and A3 8P have the speed signal on the CAN-bus and don't need this pin. This pin is only usable on RNS-E units starting with 4B0.
Without the speed signal your position will only adjust based on GPS data which doesn't work well in tunnels and similar. Your positon will also only adjust every couple of seconds instead of every seconds or less.