Because the international abbreviation codes is 3-digit, so HKG is international codes
e.g. - Japan - JPN, China - CHN
HK is normally acceptable and used for a long time. Strictly speaking, it is not an international code
HKG 只不過係 airport code......因航空管制規定, 每個機場都要有個3個英文字既 airport code 的. 故你一般會係機票或空運提單上先見到 HKG 呢組字. 至於 HK, 就係 Hong Kong 既縮寫 lor. 如 United Kingdom = UK. United States of America = USA 之嘛. 另外, 機場 code in London = LHR. 美國既 LA (羅省) = LAX. (舉 D 例你聽聽姐)