These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Incheon Intl Airport to Bangkok Suvarnabhumi Airport flight would often be abbreviated as a ICN to BKK flight. Manila Ninoy Aquino Intl Airport (MNL), Singapore Changi Airport (SIN), Ho Chi Minh City Airport (SGN), Phuket City Airport (HKT), Da Nang Airport (DAD) are other airports near this flight route and their unique identifiers/IATA codes.