Client thread
Anything you send here is visible to the client in their portal, and they can reply there or by email. Attach files to a message and they stay with it.
Internal notes
Notes here are for your team only. Clients never see them, in the portal or by email. Use them to agree an answer before you send it.
@mentions
Type @ and a name to mention someone. They get a notification straight away with a link to the message. In the client thread you can mention the client; in internal notes, your teammates.
Audit log
Every change to the order, such as a new status or a new assignee, is recorded with who made it and when.