As well as being able to post prompts in the main reply box on the thread (either using the quote option or a new reply), you also can post private prompts to users. These prompts will only show to those with admin/observer rights over the topic and the original user, they will not show to the rest of the participants.
On their initial post, click on the discuss with user icon (
) and you will get a private reply box show for you to type your comment:
If you tick the 'Email this user to alert to reply' box before posting your reply, they will receive an email to notify them about your comment; you will also receive a notification when they reply to you should you use this option.
Private prompts are clearly identifiable in the discussion:
