There is no uniform standard for bond rating, because there are subjective elements in the credit rating results. In China, as long as the regulatory authorities recognize rating agencies, their rating results according to their own standards are valid in the market. However, the symbol and meaning of rating are determined by the industry standard led by the People's Bank of China, and the People's Bank of China is the main body of rating industry management. However, the circulation of bonds in the market needs to be evaluated by rating agencies recognized by the National Development and Reform Commission, the China Insurance Regulatory Commission and the China Securities Regulatory Commission. At present, there are many AA-rated bonds in the domestic bond market, mainly because insurance institutions, as the largest investors in the bond market, are supervised by the China Insurance Regulatory Commission and can only buy AA-rated bonds from rating agencies recognized by them. Otherwise, it is useless to comment that no one voted below AA. The bond meaning corresponding to AA level basically means that the performance risk is very low.
Further reading: How to buy insurance, which is good, and teach you how to avoid these "pits" of insurance.