You might confuse SSR A codes and Mode S selective address codes . . As EXDAC said, ADS-B out will transmit the A codes set in the transponder . , Now the display of that code on the controller may vary from system to system , some will show the actual code ,most modern systems will automatically translate A7700 as " EMER" , "EMERGENCY" or some eye catchy symbol . As to your question regarding 7600, obviously you can't transmit the reason , but it does not really matter ,the procedures to follow is fixed for the controller to follow for each code . Remember that in n some real emergency cases , there is also no communications for a while until the event is over or under control , It's Aviate well before Communicate .