To many new controllers, a squawk code can appear like a list of 4 random numbers between 0 and 7 that do not mean much, however, squawk codes are the main reason why controllers can see callsigns on their radar screens through code callsign correlation. The range of callsigns is split into pre-allocated squawk codes, like for example those for emergancies, radio failures, etc, local codes and conspicuity codes.
Local Squawks
These squawk codes are typically used by approach units, as they show other units who the aircraft are being handled by, so that other units can coordinate more quickly if they need to. These squawk codes are also used by approach units to identify aircraft.
Emergency Squawks
These squawks allow ATC to know that there is an issue with an aircraft, for example in an emergency.
7500: Aircraft hyjacking
7600: Radio Failure
7700: Emergency
Conspicuity Squawks
These squawk codes allow ATC to know what an aircraft is doing, without identifying them. For example, general aviation aircraft outside of controlled airspace will squawk 7000 (VFR) or 2000 (IFR) to let controllers know that they are not being handled by another unit. Approach units can also use their own conspicuity squawk, which allows other units to know that the aircraft is on a basic service, but the approach unit does not have to identify the aircraft, which is useful if they do not have any local squawks or a radar. For example, Biggin Hill, which can only offer a basic service, will often tell VFR aircraft to squawk their conspicuity code of 7047
List of other squawk codes used in the UK
0000: Mode C failure
0033: Parachute dropping
1000: Used in where aircraft are identified by mode S as opposed to mode A
7001: Used when there is a sudden military climb out from low level routes
7004: Used for aerobatic and display code
7400: An unmanned aerial vehicle that has lost contact
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