Comments that were already read can be set as unread again. Sometimes user read a comment that was meant for other user and by reading the comment made the comment "not new".
Similar to email applications like Outlook or Gmail, comments can be marked as Unread.
To mark a comment as unread, (1) check the comment that you want to mark as unread, (2) click on the Unread button.
Alternatively, you may open the comment and click on the eye icon to unread the comment.
