ISO 3166-2:HK is the entry forHong Kong inISO 3166-2, which was published in 1998 by theInternational Organization for Standardization (ISO) to definecodes for the names of the principaladministrative subdivisions of all countries coded inISO 3166-1 (first published by the ISO in 1974).
Hong Kong itself is peculiar in that it both has its own ISO 3166-1 "top-level" code,HK (which itself has noISO 3166-2 subdivisions listed) and that it also has a code as a subdivision ofChina, asCN-HK.
Hong Kong is aspecial administrative region (SAR) ofChina, hence warranting its own "top-level" codeHK/HKG and also existing under theentry for China. This is a situation it shares with the other SAR,Macau, as well as similar territories such asPuerto RicoUS-PR /PR, andArubaNL-AR /AR.
A full list of such cases can be found at:ISO 3166-2#Subdivisions included in ISO 3166-1.
The existence of theHong Kong Flag emoji, 🇭🇰, is possible because of Hong-Kong's inclusion in the ISO 3166-1 top-level codes (see:Emoji flag sequence).