Approved plans enable control primarily by defining what is included within the approved project scope, which helps to avoid scope creep. These plans establish clear boundaries and deliverables for the project, which are essential for managing resources and ensuring that the project team remains focused on delivering the specified outputs without unnecessary expansion of the project scope.
(PRINCE2 7 Foundation Handbook, AXELOS)

Section: Volume B

To understand how user assurance will support the senior user on the project, the senior user should review the 'Role Descriptions' management product. This document outlines the responsibilities and interfaces for all project roles, including how user assurance supports the senior user by ensuring that the project's products meet the needs and requirements of the users.
PRINCE2 7 Foundation documentation on management products, particularly Role Descriptions.