A Requirements Traceability Matrix (RTM) which is used to link the test scenarios to the requirements and use cases is a required part of the Test Plan documentation for all projects. Requirements traceability is defined as the ability to describe and follow the life of a requirement, in both a forward and backward direction (i.e. from its origins, through its development and specification, to its subsequent deployment and use, and through periods of ongoing refinement and iteration in any of these phases).
Attached is a sample basic RTM which could provide a starting point for this documentation. The important thing is to choose a template or document basis that achieves thorough traceability throughout the life of the project.