A Lifeline is drawn on a Sequence diagram to represent the lifetime of a participant. The lifeline symbol consists of a rectangle forming its ‘head’ followed by a vertical line (which can be dashed) that represents the lifetime of the participant.

You can specify what the Lifeline represents within its definition.

For example, to specify that the Lifeline represents a SysML Block, go to the NamedElement tab of the Lifeline definition, click the + mark on the inherited_member_inheriting_classifier property on page 13 of that tab, and then add a class – by default its stereotype is set to <<block>>.