And ATC as well as IATA. There's a limit to the size of the flight identifier field in an ATC data tag, by international treaty/agreement (ICAO and IATA). AAANNNNZ
AAA: The 3-letter airline identifier. (though it's usually 2)
NNNN: The flight number (1-4 digits)
Z: The "operational suffix". Covers situations where a flight is so delayed, it bumps into the next scheduled flight of that number. So one of the 2 gets an additional letter in the flight strip (usually "D", "X", or "Y").

Edit: Found the ICAO reference. "not exceeding 7 alphanumeric characters and without hyphens or symbols". So AANNNNZ is right at 7 characters.