To see the history of changed statuses within the Revisions tab or send internal messages regarding a specific proposal, click the button marked on the image below to open the sidebar on the right in the Proposals page.
You will see Revisions — Status Change Logs (history of status changes, as well who and when made a change) and Messages — internal Messages (similar as comments) for a particular proposal.
In the Notifications Center, you can switch between Revisions and Messages.
Messages are visible only to the showroom users and are used as notes/comments.
It is also possible to mention other users in these messages. After you do so, your colleagues will get a notification about it.