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Use Cases - What do the sections mean?

4 important questions on Use Cases - What do the sections mean?

Preface elements would be the beginning of the use case. What preface elements are there?

Primary actors.
Secondary actors.
Pre-condition: Verifiable condition which needs to be met before the use case can be started.
Usually the pre-condition of one use-case relates to another successful use case.
Post-condition or succesguarentees: what must be true if the use case is completed successfully.

What is the main success scenario (basic flow)?

Describes the happy flow, a typical success path that satisfies the interests of stakeholders and records three kinds of steps:
1. An interaction between actors.
2. A validation (usually by the system)
3. A state change by the system (recording, modifying etc.)

What are extensions (alternate flows)?

Indication of all other scenarios, both success and failure containing two parts:
the condition
the handling

The condition to be written as something that can be detected by the system or an actor.