For reference, please see the following resources available directly from the credit rating agencies. S&P and Fitch share the same scale while Moody's operate with their own scoring with Aaa to C:
S&P: https://www.spglobal.com/ratings/en/about/intro-to-credit-ratings
Fitch: https://www.fitchratings.com/products/rating-definitions#about-rating-definitions