CSS CS Optional — Paper-I, Section-B, Topic V. Translates requirements into structured models before design begins.

🔑 The 4 Models

• DFD (Data Flow Diagram): data movement, level by level, from a single context diagram down.
• Use-Case Diagram: functional requirements from user's view.
  • Actors = users/external systems
  • Use Cases = goals actor can achieve
  • <<include>> = always incorporates another use case
  • <<extend>> = optionally extends another
• Class Diagram: structural — classes, attributes, relationships (NOT behavior).
• State Diagram: event-driven behavior — states + transitions. Best for reactive/embedded systems.

🔥 Past Papers

• 2026 Q6(b) — purpose of Use Case Diagrams.

⚠️ MCQ Trap

• Use Case Diagram = behavior (what system does)
• Class Diagram = structure (what system is made of)
• Don't swap these when comparing.