A squawk (transponder/beacon) code is a 4-digit number (each digit 0–7) an aircraft's transponder broadcasts so ATC radar can pick it out individually. Most codes — like 2443 — carry no fixed meaning. ATC assigns a discrete code to a flight for the duration of that one trip purely to tell it apart from other traffic on the radar screen, then reuses the same number for a different aircraft later. Only a small set of codes are internationally reserved with a standing meaning, shown below. If a code isn't on this list, it's simply this flight's current radar ID — nothing more.