The Chat tab in a deal's right-hand sidebar is a real-time conversation scoped to that one deal. It's created automatically the moment the deal is, so there's always a Deal Channel here.
Who's in it
Everyone currently assigned as an employee on the deal is a participant automatically — add or remove someone from the deal's Employee tab to change who's in this conversation, not from Chat itself. Vendors are never added as chat participants, even when they're linked to the deal.
Sending messages
Type a message and send it, the same as any Comms conversation — this tab doesn't support @-mentions the way the main Comms page does. Select Open in Comms to view the same conversation full-screen on the Comms page, where the full feature set (search, mentions, member list) is available.
System messages
If Comms Automation is turned on for your team, automatic "Deal activity" notices — status changes, employee or vendor changes, and more — post into this same conversation, clearly labeled so they don't read like a person joined the chat.
This is a different thread from the deal's Comments tab — Comments is a lighter, internal note thread; Chat is the full real-time conversation.