This feature allows server side applications to send system notifications (aka notifications bubbles) to the client.
It is supported on all platforms and controlled by the notifications configuration option.
python-notify or python-dbus (the exact name of the packages required vary)MS Windows XP:

MacOS 10.10.x:

Gnome-shell:

-d notify,dbusyou can also test notifications forwarding using the dbus interface or xpra control, ie:
xpra control :100 send-notification "hello" "world" "*"
will send the message to all clients.