"The protection of Controlled Unclassified Information (CUI) resident in nonfederal systems and organizations is of paramount importance to federal agencies and can directly impact the ability of the federal government to successfully conduct its assigned missions and business operations. This publication provides federal and nonfederal organizations with assessment procedures and a methodology that can be employed to conduct assessments of the CUI security requirements [...]. The assessment procedures are flexible and can be customized to the needs of the organizations and the assessors conducting the assessments. Security assessments can be conducted as self-assessments; independent, third-party assessments; or government-sponsored assessments and can be applied with various degrees of rigor, based on customer-defined depth and coverage attributes. The findings and evidence produced during the security assessments can facilitate risk-based decisions by organizations related to the CUI requirements."
NIST Special Publication 800 -171A
National Institute of Standards and Technology: https://www.nist.gov/