Flagging allows a student to mark a particular piece of feedback that they disagree with or find incorrect.
The teacher will be alerted that a piece of feedback has been flagged and is then able to review the flagged feedback and make a final decision on what evaluation to give.
The student whose feedback was flagged will not get notified, but they will be able to see that their feedback got flagged and why if they navigate to the particular piece of feedback.
To find the flags for an assignment, go to the assignment page and then click on Results tab.