ITPEC FE Morning October 2021 Question 45

Use case diagram — it shows the actors outside the system and the functions (use cases) they use.

Its elements are few:

• •Actors (stick figures) — users or external systems that interact with the system.
• •Use cases (ovals) inside the system boundary — the functions the system provides.
• •Lines between them — which actor uses which function.

It is drawn during requirements definition to agree on the system's scope with the users.

Answer (d)

Why not others:

• •(a) Activity diagram — the flow of actions and decisions in a process, like a flowchart.
• •(b) Class diagram — the static structure: classes, attributes, operations and their associations.
• •(c) Sequence diagram — the order of messages between objects over time in one scenario; it details how a use case runs, not which functions exist.

Key rule: Each UML diagram answers one question: who uses which functions (use case), what the structure is (class), in what order objects talk (sequence), and what flow the steps follow (activity).

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