You can setup custom codes by clicking the SETTINGS link in the upper right corner, select any gradebook, then the Go button under Custom Codes section.
Create a new code
Code: This is the value you will enter as a score. Custom codes are case insensitive and should be 3 or fewer characters since that's about all you'll be able to see in the cells where you enter the scores. We do not recommend using numbers as unless combined with another character.
Definition: Plain English explanation of the code.
Value: Take your pick. If you choose Percent, you must also enter a numerical value in the Percent input field.