In UML diagrams, such as sequence and communication diagrams, you
can create lifelines to represent objects in a system.
You must have a sequence or communication diagram open.
If you are creating a lifeline in a sequence diagram, the lifeline
is aligned vertically with other lifelines in the interaction, and extends
downward to cover the length of the interaction frame.
Note: You can move a
lifeline to make a diagram easier to read. To move the lifeline left or right
in a sequence diagram, drag the selected lifeline horizontally to a new location
in the interaction frame.
Tip: You can also assign an existing
element type in the navigation view to a lifeline. In the navigation view,
click a class, interface, or data type and drag it onto a lifeline in the
interaction frame.