What does a sequence diagram illustrate?

A sequence diagram is a type of interaction diagram in UML (Unified Modeling Language) that specifically focuses on the timing and order of interactions between objects or components within a system. It illustrates how objects interact with each other through messages over time, detailing the sequence of events in a particular scenario. This is crucial for understanding the flow of control and the timing of messages exchanged during a particular use case, making it a valuable tool for system analysts in capturing dynamic behavior.

In contrast, the other choices do not align with the primary purpose of a sequence diagram. Project milestones relate to planning and scheduling rather than the interactions of system components. Process flow generally refers to the overall flow of activities or tasks within a system, which can be represented through different types of diagrams such as activity diagrams or flowcharts. User roles focus on the permissions or responsibilities assigned to different users in a system and would be represented in different models, like use case diagrams or role-based access control models.