There are several levels of bond rating.
Bonds below BB grade are speculative: although speculative bonds have high yield, they are risky, and bond issuers may not repay their debts at maturity. Starting from 1982, in order to measure the credit rating of bond issuers more accurately and meet the different needs of fundraisers and investors in the securities market, Moody's Company of the United States added the numbers 1, 2 and 3 after AA to B to indicate various ratings. Such as AAl, AA2, AA3, etc. Obviously, the credit rating of AA 1 is higher than AA2, which means that the investment risk is low.