As Sleepybhudda mentions probably the difference between precision and non precision but yet still vertically defined path. Back when i was still on the 737 it showed G/P on the FMA in an IAN non-precision approach (FMC computed glidepath) and G/S for precision approaches (ILS/GLS). Dunno if EASA uses something similar, but it does make sense in a way. Sadly now on the bus we do not have something as easy to use as IAN, although i hear the newer ones do have it.