AC-21 Information sharing
What the standard requires
a. Enable authorized users to determine whether access authorizations assigned to a sharing partner match the information’s access and use restrictions for [information-sharing circumstances] ; and b. Employ [automated mechanisms] to assist users in making information sharing and collaboration decisions.
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-21 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.