Generally speaking, the characteristics of main insurance and additional insurance are that the main insurance is the main responsibility, and some can be purchased separately, while the additional insurance cannot be purchased separately, and it must be attached to a main insurance. In addition, the main insurance is usually expensive, while the additional insurance is very cheap, so there is usually a proportional limit between the main insurance and the additional insurance, which is also to reduce the risk cost of the insurance company for the additional insurance.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.