These are just IATA (International Air Transport Association) codes. They are used in aviation to identify and differentiate airports. That’s why a Corpus Christi Airport to Istanbul Airport flight would often be abbreviated as a CRP to IST flight. Istanbul Sabiha Gokcen Airport (SAW), Bursa Yenişehir Airport (YEI), İzmit Cengiz Topel Airport (KCO), Tekirdağ Corlu Airport (TEQ) are other airports near this flight route and their unique identifiers/IATA codes.