Mode A transponders are sometimes referred to as Mode 3/A transponders. According to NATO STANAG 4193 (not publicly available, summary from Wikipedia), the "3" in that description refers to the mode in the Identification of Friend or Foe ("IFF") system:
- Mode 1 – 2-digit octal "mission code" that identifies the aircraft type or mission.
- Mode 2 – 4-digit octal unit code or tail number
- Mode 3/A – 4-digit octal identification code for the aircraft, assigned by the air traffic controller (the only mode shared with civilian aircraft)
- Mode 4 – 3-pulse reply, delay is based on the encrypted challenge
- Mode 5 – Cryptographically secured version of Mode S and ADS-B GPS position.
In flight school I was taught that Mode C was an improvement on Mode A through the addition of data fields that provide aircraft altitude. I extrapolated that to Mode S being an improvement on Mode C through the addition of yet other data fields and making the interrogation targeted at a specific aircraft, and Mode ES/ADS-B being a further improvement through automatic broadcasting.
I have, however, never seen Mode S referred to as Mode 3/S. So, is Mode S a variation within the Mode 3 "protocol" or is it a stand-alone mode outside of the five IFF modes?