Flagging and commenting on questions is really useful when you're going back and forth with a client on a couple unanswered questions in your Smart Form.
To flag a question, click the flag icon in the top right corner of a question. You'll see it turn orange. You can unflag the question by clicking it again. You can then select "Flagged Questions" from the "More Actions" dropdown in the top right corner of your page to display only flagged questions to anyone using the Smart Form. If you would like, you can then resend the Smart Form invitation to your client so that they know to review it.
To comment on a question, click the comment icon in the top right corner of a question. Anyone accessing the Smart Form can see your comment. You can comment the question by clicking the comment icon again.