Great question - SUB to BKI just means Surabaya Juanda Airport to Kota Kinabalu Airport. Now you’ll know what these unique codes when you look for flights! Kuching Airport (KCH), Bandar Seri Begawan Brunei Intl Airport (BWN), Miri Airport (MYY), Tawau Airport (TWU), Balikpapan Sepinnggan Airport (BPN) are other airports near this flight route and their unique identifiers/IATA codes.