What is the primary purpose of a sequence diagram?

The primary purpose of a sequence diagram is to express behaviors and event occurrences over time. Sequence diagrams are a type of interaction diagram that detail how objects operate with one another and in what order these interactions occur. They visually represent the sequence of messages exchanged between different elements, reflecting the dynamics of a process or a scenario across time. This chronological aspect allows stakeholders to understand the flow of control and data between various components, providing clarity on how individual actions contribute to the overall system behavior.

In contrast, marking the start of activities, indicating the ownership of actions, and terminating a flow of control are functions that may be associated with other types of diagrams or specific notations within larger models, but they do not capture the essence of what sequence diagrams are fundamentally designed to convey. Sequence diagrams focus specifically on the timing and sequences of messages rather than the initiation, ownership, or termination of activities in a system.