In the Submissions console for a lab, Instructors and Graders can add comments to each criterion of the grading rubric after a learner makes a submission.
Adding Comments
From your course page navigate to the Dashboard tab. Then select Submissions below.
Select View from the row of a learner to view their submissions.
Navigate to the review comments on the right side. Select the word bubble next to the students's submissions to leave a comment and select save.