Requirements with Alternatives

Question

In the project I am currently working in, the requirements is on this form:

1. The system must ...
2. The system must ...

It works fine as long as there are no alternative scenario. But how should I write if the system work differently depending on what happens?

if ScenarioA Then do This
elseif ScenarioB Then do That

Answer 1

Why don't you try User Story?

User Stories (opposed to requirements) are brief statements of intent that describe something the system needs to do for some user.

As a user closing the application, I want to be prompted to save anything that has changed since the last save so that I can preserve useful work and discard erroneous work.

1. For scenarioA do this
2. For scenarioB do that

1. As a user doing scenarioA, I want this, so that I can open my application.
2. As a user doing scenarioB, I want that, so that I can close my application.

Answer 2

You are mixing requirements and design (more specifically, use cases). Requirements describe the high-level functionality that the system should be able to provide. Use cases are derivatives of the requirements. So your requirements can be:

1. For scenarioA do this
2. For scenarioB do that

Note, the requirements are context-free and essentially describe capabilities.

And from these requirements a use case (or even several) describing a certain dynamic behavior can be defined:

a. Step 1
b. Step 2
...
n. [ScenarioA] do this
n1. [ScenarioB] do that (alternate path)
...

Here, the n and n1 steps are defined in scope of the use case context and have not meaning otherwise.