What kind of identification code is this format? (AAA0000)
That format is the flight id. The first three characters are the ICAO operator code which identifies the airline. The rest of the characters are unique to that flight. The first character after the ICAO operator code is always a number, after that both numbers and letters are allowed. The maximum length of the flight id is 7 characters.

The flight id is transmitted to Mode S radars by the transponder and also broadcast on ADS-B. It is used to correlate a flight to a flightplan. 

The flight id is also the basis for the callsign used in radio communications. The ICAO three letter code represents the airline's callsign, the rest of the code is pronounced in the ICAO alphabet.