Requirements are simply an inventory of events, conditions, or functions that need to be satisfied and tracked by an application.
Requirements are described in various forms. Some are non-functional while others articulate specific business rules that must be satisfied. They can also be typed directly into use case text, but if requirements can be shared by a number of use cases, the rule of thumb is to separate them into individual requirements and link them to the use case.
Typically, requirements are specified in an Application Profile, created by the Enterprise Application Accelerator, and made available for update as instances of Rule-Application-Requirement rules as a project evolves.
They also are referenced in application, use case and and flow rules.
For details about working with requirements, see the Pega Developer Network articles listed under the Implementation Methodology knowledge category.