What does the lifeline represent in a sequence diagram?

The lifeline in a sequence diagram is a fundamental concept that represents a single instance participating in an interaction within the system. Each lifeline corresponds to an object or a participant that engages in the interaction depicted in the sequence diagram. The vertical dashed line that extends down from a rectangle (representing the object) visually indicates the existence and lifespan of that object over time during the interaction.

In the context of a sequence diagram, the lifeline is crucial for understanding the flow of messages exchanged between objects. It helps to clarify which object is sending or receiving messages and shows the temporal sequence of those exchanges. By clearly establishing the role of each lifeline, you can effectively trace how different parts of the system communicate, which is essential for modeling use cases and understanding system behavior. Thus, identifying a lifeline as representing a single instance is key to interpreting the interactions correctly.