Michely,
You are correct that the PMBOK Guide does not specifically say how you use the Requirements Management Plan after you create it. It just says that you need it. Remember though, that the PMBOK Guide is a GUIDE and not a step-by-step project management methodology / checklist. It doesn't tell you everything.
The RMP, as most other plans as well, is meant as a reference document. It documents your strategy and tactics for how you will be planning, tracking and reporting your requirements. It also tells you how changes to requirements are initiated and approved.
The idea is that throughout the project, whenever you are working on your requirements documentation, you need to follow the plan. For instance, if you have a change to a requirement the plan should be able to tell you what change request form you need to fill in and who will be approving it. It defines your the processes your project follows in regards to requirements management.
Until Next Time,
Cornelius Fichtner, PMP, CSM
President, OSP International LLC