The agent IM system is used to send notifications about @mentions. When someone @mentions you in a Ticket Note, you can either respond to them via IM or by replying with another Note with an @mention. The advantage of using Notes to discuss a Ticket is that they are kept as part of the Ticket history, so it’s easier for other Agents to find your discussion and see it in context. Users do not have access to the Notes section of the Tickets.
To @Mention an agent in a Note: