Mostly: trade relationships other countries have that are specifically relationships with Hong Kong, rather than with China generally; where, sometimes, those other nations aren't even interested in trading with China generally, only with Hong Kong specifically.
To the degree that China wants to maintain these relationships (and capture the GDP they represent), they treat Hong Kong as a peer. To the degree that China has built replacements for these relationships, they treat Hong Kong as a vassal.
To the degree that China wants to maintain these relationships (and capture the GDP they represent), they treat Hong Kong as a peer. To the degree that China has built replacements for these relationships, they treat Hong Kong as a vassal.