Each object that has a score in the Infer Score field also has a value in the Infer Rating field. The rating is determined by a set of configured Thresholds. For example, by default, Objects that score over 80 have Lead Rating "A", scores between 50 and 80 are "B", Scores between 30 and 50 are "C" and scores below 30 are "D."
To configure the Score names and Thresholds, go to Setup, then to Installed Packages, and go to the Infer Scoring AppExchange Package Configuration page, to Score Settings.
After filling in the desired values, click Save.