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 at the top.
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.