If you listen to any air traffic control (ATC) recordings for departing aircraft, controllers and pilots will constantly refer to squawk codes.

These are the transponder codes used to identify an aircraft to ATC. The code can be changed from the specified one assigned to that flight by ATC in certain circumstances - most importantly in the event of an emergency when it is switched to 7700.

Läs mer här:

https://simpleflying.com/squawk-7700-meaning/