Apple Remote Desktop allows you to communicate with users of Apple Remote Desktop client computers using text messaging. You can use text messages to give instructions or announcements, to collaborate remotely, or troubleshoot with users. There are two types of text messaging:  one-way messages and two-way interactive chat. Text messages and chat are available only to Apple Remote Desktop client computers; they are not available to VNC client computers.

See:

• Sending One-Way Messages.
• Interactive Chat.
• Requesting Administrator Attention.
• Canceling an Attention Request.
• Viewing Attention Requests.