An end-user creates a ticket via email and then the technician responds via an email reply and the end user does not receive a notification.
However, if the tech adds the comment via the console then the "Comment" notification email is sent. In both cases the ticket is updated with the comment.
Customer can workaround this by going to Service Desk | Configuration | Queue Detail and within the Email on Events section select Any Change and check Owner and Submitter. This leads to unwanted email notifications.