ISO 3166-2:HK

ISO 3166-2:HK is the entry for Hong Kong in ISO 3166-2, which was published in 1998 by the International Organization for Standardization to define codes for the names of the principal administrative subdivisions of all countries coded in ISO 3166-1.
Hong Kong itself is peculiar in that it both has its own ISO 3166-1 "top-level" code, and that it also has a code as a subdivision of China, as.
Hong Kong is a special [administrative regions of the People's Republic of China|special administrative region] of China, hence warranting its own "top-level" code / and also existing under the entry for China. This is a situation it shares with the other SAR, Macau, as well as similar territories such as Puerto Rico /, and Aruba /.
A full list of such cases can be found at: ISO 3166-2#Subdivisions [included in ISO 3166-1].
The existence of the Hong Kong Flag emoji, ??, is possible because of Hong-Kong's inclusion in the ISO 3166-1 top-level codes.