Based of this information provided I would suggest the following:
- Regional Census = Provincial-level divisions
- City Census = Municipal-level divisions
I assume the that Provincial-level divisions (provinces, special autonomous provinces, special cities, metropolitan cities and special autonomous cities) are all mutually exclusive and have no overlap. E.g. a special city is not within a province. I suspect this may not be the case.
I’d go with what culturally makes sense. Ask your friends which census they’d expect to find Busan, Ansan, Seoul or Sejong in - Regional or City.