Scope Management Plan

It is part of the project management plan and provides guidance on how the project management team will define, document, verify, manage, and control the project scope.
It depends on the project and can be formal or informal, detailed or high level, etc.
The scope management plan is iterated during project planning.
There is no perfect Scope Management Plan as it is the one plan that could differ significantly from the PMI approach but still meet the goal of helping the PM achieve the project objectives.
Note that the stakeholder needs must be balanced by the PM in order to finalize the product and project scope.
As a seller, you may be just clarifying the project scope instead of documenting it since the buyer typically documents the project scope unless that process is outsourced as a separate project.
The PM must ensure that all requirements are aligned with the project objectives and meet the business needs of the project as defined in the project charter.