The tab shows all messages exchanged with the user inside one support request (ZZP) — the user's and yours — in chronological order, with the {Date} and {Message} columns. Above the list is a search box over the message text.
New messages to the user are entered here as well. All communication on one topic takes place inside the same request — see One problem – one request.
The {Internal message} checkbox in the message dialog marks a message intended for the team only: it is sent to the person in charge and the associates, and the user does not see it.