What Happened To Special Publication 800-53A Revisions 2 And 3?

Revision numbers between NIST Special Publications 800-53 and 800-53A were misaligned from the start because the initial publication of SP 800-53A did not occur until after the publication of SP 800-53, Revision 2. When SP 800-53, Revision 3 was published, SP 800-53A was updated to Revision 1 for consistency with the updates to SP 800-53. This revision number mismatch created ongoing uncertainty and confusion regarding which revision of SP 800-53 was consistent with which revision of SP 800-53A. To reduce this uncertainty going forward, revision numbers 2 and 3 have been skipped for SP 800-53A, and this version of SP 800-53A has been given revision number 4 since this version is consistent with the updates to SP 800-53, Revision 4. Future revisions of SPs 800-53 and 800-53A will maintain the revision number consistency.