In principle, administrative regions above the prefecture level (including prefectures, cities, prefectures and leagues) are defined as the overall planning units, and counties (cities) with a certain population can also be used as the overall planning units. All units and employees should participate in the basic medical insurance in their overall planning areas in accordance with the principle of territoriality, implement unified policies, and implement unified collection, management and use of basic medical insurance funds. Enterprises and employees with relatively large production mobility such as railways, electric power and ocean transportation can participate in basic medical insurance in different places in a relatively centralized way. In principle, Beijing, Tianjin, Shanghai and Chongqing are the four municipalities directly under the Central Government.

Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.