In HappyFox, the subject line of a support request email is automatically converted into the title of the ticket. Whenever you respond back to the support query, the email carries the same subject with the respective reply tags.
In some cases, you might need to address a typo in the subject line or edit it to reflect the original issue. This can be done by using the Change Reply Subject feature in the rich text editor.
Do note that only the subject of the email gets changed and not the original ticket title!