The key thing to ask yourself here is Would you call Hong Kong a special administrative region without capitalization if the PRC had not come up with that term Sure the sentence would make some sense if Special Administrative Region was converted to its improper form although most readers would have little to no idea of what a special administrative region actually is By making the noun proper you take away most of the burden of conveying the noun s meaning to the reader I m in favor of describing Hong Kong as a largely self governing territory which essentially is a good definition of Special Administrative Region which really is little more than a somewhat biased title made up by the PRC