Intro

Describe how an actor achieves a goal using the system. This is described in terms of scenarios.
Interests of stakeholder are protected by the system.

Actor

An actor is someone or something external to the system and who interacts with the system.

A scenario is a description of a sequence of actions that is a way of achieving a goal.
The main success scenario is the usual/main/straightforward way of achieving the goal.
All scenarios have
- A precondition : required condition before starting.
- Interaction steps
- Postcondition :
  - Guaranteed state of world once use case exits. successful precondition holds for successful exit.
- A minimal postcondition: Something that guarantees to protect stakeholder interests.

For each step of main success scenario, think of:
Secondary Scenarios:
- Alternative Scenario: A different way of achieving success (the goal)
- Exception Scenario: Something that can go wrong - and can lead to a different postcondition.

A step in the scenario of a use case that is detailed - and might be another use case by itself.
The main use case "calls" the inclusion use case.
For e.g. use case for "Edit Document" - might have a step called "Modifies Text" which might be a use case by itself.

Sort of an interruption to to the Main use case.
For e.g. if the main use case is "Check Balance" and if the balance > 5000$, the "Enrol as privilege customer" use case can kick in.
Note that the main use case has no control over the special use case.

Is a summary of the functionality of the whole system.
Gives an overview of the relationships between all the actors and all the use cases.
Not meant to record details of use cases - for that text is best, but rather as a sort of visual guide to the various use cases in the system.