The project scope management plan refers specifically to the input/output mechanism that consists of a formalized document that is used for the purposes of detailing exactly how the project scope will be defined, what means will be undertaken to develop the project scope, how the project scope will ultimately be verified, and exactly how any and all components of the work breakdown structure will be ultimately created and defined. The project scope management plan also will provide information and assistance in determining exactly how the actual scope of the project will ultimately be controlled in the management process by the project management team and or the project management team leader. The actual project scope management plan, as with most project management components, can be a very formally written document, or it can also be a much more informally written document. The detail level can vary wildly as well, depending on exactly what the needs of the project dictate.
This term is defined in the 3rd and the 4th edition of the PMBOK.