Quiz Entry - updated: 2026.07.14
What are the four major elements of a UML Use Case Diagram (UCD)?
Just four: Actors (who), the System boundary (what's in scope), Use Cases (what it does), and the Relationships (lines) connecting them.
A UCD is intentionally minimal — these four are all you get, which is why it's such a fast, low-jargon way to agree on scope with non-technical stakeholders early in a project. Each element answers one question:
| Element | Answers | Description | Visual |
|---|---|---|---|
| Actor | Who? | Entity outside the system that interacts with it (user, device, external system) | Stick figure |
| System | What's in scope? | The boundary separating what you're building from the outside world | Rectangle |
| Use Case | What does it do? | A service or function the system offers | Oval/ellipse |
| Relationship | How do they connect? | Lines tying actors to use cases (and use cases to each other) | Lines/arrows |
Why it matters for security: the system boundary is also the trust boundary, and every actor-to-use-case line is a place where untrusted input crosses into your system — so the same four-element sketch that scopes the project also maps its initial attack surface.