If the credit rating of corporate bonds is AAA, its solvency is extremely strong, basically unaffected by the adverse economic environment, and the default risk is extremely low. If the grade is AA, its ability to repay debts is very strong, and it is not affected by the adverse economic environment, and the risk of default is very low. A-class companies have strong solvency, are more vulnerable to adverse economic environment, and have lower default risk. If the rating is BBB, its solvency is average, which is greatly affected by the unfavorable economic environment and the default risk is average.