NIST SP 800-53A Rev. 5 53A-3.2.6: Finalize the Assessment Plan and Obtain Approval
Fixes the plan and secures approval to execute before assessment begins, so scope and rigour are agreed rather than negotiated during fieldwork.
What else in your programme already covers this
This control maps to 15 controls across 10 other frameworks. If you already hold one of them, the evidence you collected for it is the starting point here rather than new work.