AC-20(1) Use of external systems | limits on authorized use
What the standard requires
(a) Verification of the implementation of controls on the external system as specified in the organization’s security and privacy policies and security and privacy plans; or (b) Retention of approved system connection or processing agreements with the organizational entity hosting the external system.
Quoted verbatim from NIST SP 800-53 Rev 5 (OSCAL catalog), which is in the public domain.
Cross-framework mappings
No cross-framework mapping is seeded for this control. That is stated rather than filled in: a mapping nobody authored is worse than a gap you can see.
Track AC-20(1) with evidence behind it
In CMDash this control is a row with an owner, a status, uploaded evidence kept with its full history, and an export an assessor can read.