What is the format of the airport codes used in an ACARS OOOI message? How are the departure and arrival airport codes in the OUT, OFF, ON and IN (OOOI) fields identified by ACARS and transmitted to ground systems?

For example, if my aircraft is diverted unexpectedly to Bangkok because of some emergency, will ACARS send the arrival airport code as BKK in the IN message? If so, how will it be identified?
ACARS messages, unsurprisingly, use IATA airport codes.

See ARINC 618, 619, 620 etc.




- ACARS, Dr. Maarten Uijt de Haag, Ohio University