The system border, if present, is connected to all instances
participating in the execution that do not have a special line of
their own. In other words, a message is drawn between two instance
lines if both the sending and receiving instances have instance lines
in the animated sequence diagram.
If either the sending or receiving instance does
not have a line, messages can be displayed between the instance that
has a line and the system border:
- If the animated sequence diagram includes a system
border, the message is drawn between the instance that has a line
and the system border.
- If the animated sequence diagram does not include
a system border, the message is not displayed in the diagram. In other
words, removing the system border prevents the display of messages
to or from instances that are not explicitly part of the execution
sequence.