Correspondence

Correspondence is a message added to a Ticket that involves the Requestor and any CC on the ticket, /i.e./ the external customer. Sometimes this is called a Reply.

Correspondence is different in this aspect from a Comment, which is only visible to internal people.

The difference can be seen from the words themselves: the word Correspondence is much more formal than the word Comment, and a piece of Correspondence is normally more formal than a Comment.