Why do programmers get Halloween and Christmas mixed up? Because OCT(31) = DEC(25)
Use cases can be related to each other. For example, a subfunction use case such as Handle Credit Payment may be part of several regular use cases, such as Process Sale and Process Rental. Organizing use cases into relationships has no impact on the behavior or requirements of the system. Rather, it is simply an organization mechanism to (ideally) improve communication and comprehension of the use cases, reduce duplication of text, and improve management of the use case documents.