See Accessing the Factor Rationale Page to navigate to the proper section. Once in the proper sub factor, you can add individual lines of rationale for each level of the sub-factor. To add rationale, click the green button "+ add additional rationale line within level", an empty text box will appear and you can enter your rationale within the box. The rationale is automatically saved when you leave the text box. To edit an existing line of rationale, click into the rationale line and make changes accordingly.
See also:
Turning Half Levels On / Off
Adding Multiple Lines of Rationale to the Same Level
Add sub factor Instructions
Why cant I change anything the in the Point Table / Factor Rationale sections