What are Mode 3/A squawk codes?

Four digits, not a callsign

Mode A is the civil name; Mode 3 is the military name for the same compatible mode. That is why it is written Mode 3/A. The code uses four digits from 0 to 7: 8 × 8 × 8 × 8 gives 4,096 combinations. It is not a permanent aircraft identity or a flight number.

Where the code comes from

ATC assigns codes under allocation arrangements; designated codes also have specific uses. Allocation varies by region and purpose. A code does not universally identify a particular airline, aircraft type or military mission.

Height is a separate piece of information

Mode C supplies pressure-altitude information. Read the mode and the data carefully rather than assuming the four-digit squawk itself encodes height. In Air Defender, use the in-game Mode 3 reference for the simulation’s assignments.

Use the game’s period reference

1. During an ordinary session, close the Escape menu if it is open. Click HELP on the bottom menu, then Mode 3 in the Guide. Some training lessons hide the bottom menu; follow the lesson’s instructions instead.
2. The page is headed UK SSR CODE ALLOTMENT PLAN. Enter a code or an authority name in Search. Read the Call ID, Codes and Controlling Authority / Function columns. A Call ID is the identifier used in an ASMA controller call, not a button on this list. -- means the entry is not callable.
3. If no rows match, clear Search to show the list again. A match identifies a code allocation in the game; it does not by itself identify the aircraft or prove that a controller has confirmed its flight.
4. Use this as the simulation’s reference, not a source of present-day operational allocations. A code such as 3701 is neither a height nor a globally unique aircraft identity. Continue with the ATC identification checks.