In a sequence diagram, drag a Lifeline shape onto the drawing page near the object lifeline on which you want to show conditionality.
Glue the endpoints of the Lifeline shape to connection points on the Object Lifeline shape. To extend the conditional lifeline, drag a control handle on the Lifeline shape.
Double-click the Lifeline shape to open its UML Properties dialog box where you can add a name and other property values to the lifeline.
Tip: You can also glue the endpoints of a Lifeline shape to connection points on an Activation shape associated with an object lifeline.