CUI is Controlled Unclassified Information (CUI), this is information “that requires safeguarding or dissemination controls pursuant to and consistent with law, regulations, and government-wide policies, excluding information that is classified under Executive Order 13526, Classified National Security Information, December 29, 2009, or any predecessor or successor order, or the Atomic Energy Act of 1954, as amended.”
Source: Executive Order (2010), EO 13556 (adapted)
CUI must be identified and marked by the government, and it is information that requires safeguarding.