Just copied the definition of Cat A to F from The Avifauna of Hong Kong:
Cat A - Species that have been recorded in an apparently wild state in HK
Cat B - Species that are considered to have probably occurred in HK in a wild state, but for which the possibility of escape or release from captivity cannot be satisfactorily excluded.
Cat C - Southeast China breeding species, the established or formerly established in HK population of which is considered to be derived from captive stock, but which probably occurred in HK prior to anthropogenic habitat changes.
Cat D - Extralimital species which, although originally introduced to HK by man, maintain or did maintain a regular feral breeding stock without necessary recourse to further introduction.
Cat E - Species for which all published HK records are considered likely to relate to birds that have escaped or have been released from captivity.
Cat F - Species for which all published HK records must be regarded as doubtful because of possiblity of mistaken identification.
