When you've opened a Ticket, you can send private messages that you and your colleagues can see in the Tickets feed, but that the requester can't see. This feature is useful when you and your colleagues want to communicate internally about a Ticket.

To send private messages, you click the toggle switcher named Private note. The message field then turns yellow and the toggle switcher goes from grey to purple.

After clicking Send, your private message lands in the Tickets feed. The message field is light yellow also in the feed, which indicates that it's a private message. Next time you answer the requester, the yellow message field/private message won't be visible for the requester.