The Requirements Management Plan describes the level of governance with regards to the Business Requirements. It describes the operational procedures for the requirements engineering including the operational guidelines for documenting the Business Requirements and the use of tooling. So it is not limited to the theoretical description of the approach, it also enable the day to day operations on the aspect of requirements.
Note that implementation of the operational procedures described in the Requirement Management Plan are part of the responsibilities of the Engagement Manager and therefore not covered by SEMBA.

The Requirements Management Plan enables an agreement on the level of management and governance of the requirements.

If the client has a standard approach for requirements engineering, then it will be checked if it is suitable for the engagement.