Develop, document, and maintain a system security plan describing the system boundary, environment of operation, security requirements implementation, and connections to other systems.
NIST SP 800-171 Rev 3 03.15.02 is one control. If you already hold one of the frameworks below, a reviewed crosswalk already says how much of NIST SP 800-171 Rev 3 your existing evidence covers. Hold PCI DSS 4.0 and 69 of 97 NIST SP 800-171 Rev 3 controls already carry evidence.
Each report names every control your existing framework evidences, every one it does not, the reasoning behind each claim, and the claims that were argued against and rejected. 0 were rejected on the PCI DSS 4.0 pair alone.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.