In a system sequence diagram, lifelines indicate the period of time when the participant is active in the interaction.

A system sequence diagram is a visual representation of the interactions between the system and external actors or other systems. It is used to model the flow of messages and actions in a system.

In a system sequence diagram, lifelines represent the participants or actors involved in the interaction. Each lifeline represents an instance of a class or an actor in the system. The lifeline shows the period of time during which the participant is active in the interaction. It starts when the participant is created or becomes active and ends when it is destroyed or becomes inactive.

Lifelines are depicted as vertical lines with a box at the top representing the participant. Messages between lifelines are shown as arrows between the lifelines, indicating the flow of communication. Lifelines can also have activation bars, which represent the period of time when the participant is actively processing a message or performing an action.

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