Requirements specify conditions the solution MUST meet – in contrast, the solution architecture document specifies conditions the solution WILL meet – or already meets, depending on which phase the development is in at the time.
As such, the requirements document’s use depends on whether you’re using it to set conditions that an external supplier must meet, or if you’re using it to capture conditions that cut across multiple use cases – for example business rules.
Start writing this document in the outline phase.
List all requirements.
List all use cases.