It's a fair question. I was going to answer that a read of the appropriate manual should answer the question. But if the manual as suggested by No Turning Back is correct, it is of no real use. IMHO the flags VOR, VLOC and GPS are source indicators. I reckon that there will also a source selector panel somewhere else and it will say something like NAV1 and GPS.
As an aside, I never understood any of the electric gismos in any powered aircraft untill I did my IMC rating. I also felt that I had no need and with hindsight I think I was right. TLAR is still the best nav system known to man.