Message pane
The Message pane contains the elements that make up the interaction. In the
object pane, system borders and instances are displayed as instance line, which are vertical lines
with a box containing the role name.
Messages, such as events,
operations, and timeouts are generally shown as horizontal and slanted arrows.
The messages appear in sequence as time advances down the diagram. The vertical distance between points in time indicates only the sequence in time and not any time scale.