As pointed out by Robert, the document says it all:
Maintainability Rating (sqale_rating)
(Formerly the SQALE rating.) Rating given to your project related to the value of your Technical Debt Ratio. The default Maintainability Rating grid is:
A=0-0.05, B=0.06-0.1, C=0.11-0.20, D=0.21-0.5, E=0.51-1
The Maintainability Rating scale can be alternately stated by saying that if the outstanding remediation cost is:
- <=5% of the time that has already gone into the application, the rating is A
- between 6 to 10% the rating is a B
- between 11 to 20% the rating is a C
- between 21 to 50% the rating is a D
- anything over 50% is an E