# Use case diagram

Use case diagrams are unfashionable and still useful for one job: agreeing scope with people who do not read technical diagrams. The boundary line is the whole point.

Keep the use cases at the level of goals, not steps.

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## When to draw one

* Scoping conversations with non-technical stakeholders.
* Requirements documents where the actors matter.
* Explaining what a system does without explaining how.

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## What is on the diagram

| **Element**     | **Meaning**                             |
| --------------- | --------------------------------------- |
| Actor           | A person or system outside the boundary |
| Use case        | A goal the system helps achieve         |
| System boundary | The box: what is in scope               |
| Include         | One use case always uses another        |
| Extend          | One use case sometimes extends another  |

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## A worked example

Actors, a boundary, and include and extend relationships:

![Use case diagram](https://help.gocapable.com/images/att1418297357.svg)

The source, which you can paste into a new diagram and edit:

```
@startuml
left to right direction
actor Customer
actor "Support agent" as Agent
actor "Payment provider" as Pay

rectangle "Online shop" {
  usecase "Browse catalogue" as UC1
  usecase "Place order" as UC2
  usecase "Pay for order" as UC3
  usecase "Apply discount code" as UC4
  usecase "Refund order" as UC5
}

Customer --> UC1
Customer --> UC2
Agent --> UC5
UC2 ..> UC3 : include
UC4 ..> UC2 : extend
UC3 --> Pay
@enduml
```

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## A few things that catch people out

* Include means always; extend means sometimes. Swapping them changes the meaning.
* Actors go outside the boundary. An actor inside it is a component, not an actor.
* Keep use cases to goals. "Click the save button" is a step, not a use case.

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## Related

[Activity diagramFlow with forks, joins and partitions.](https://help.gocapable.com/diagrams/activity-diagram.html)

[State machine diagramWhat states exist, and what moves between them.](https://help.gocapable.com/diagrams/state-machine-diagram.html)

[UML diagramsBack to the UML overview.](https://help.gocapable.com/diagrams/uml-diagrams.html)

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_Four types cover most of it._