As an analysis step, often it is desirable to capture simple system requirements. These are eventually realized by Use Cases.
In the initial requirement gathering phase, cataloging requirements can be achieved using the Requirement extension on a Custom diagram.
Requirements can also be aggregated to create a hierarchy. The diagram below illustrates how this might be done.
A requirement that a user can log into a website is implemented by the Login use case, which in turn is implemented by the Business Logic, ASP Pages and Login Web Page. Using this approach, you can easily model quite detailed and complex dependencies and implementation relationships.