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See Security Control Item.
NISTIR 8011 Vol. 1
All or part of a SP 800-53 security control requirement, expressed as a statement for implementation and assessment. Both controls and control enhancements are treated as control items. Controls and control enhancements are further subdivided if multiple security requirements within the control or control enhancement in SP 800-53 are in listed format: a, b, c, etc.
NISTIR 8011 Vol. 1 under Security Control Item