According to DFARS clause 252.

DFARS clause 252.204-7012 establishes the safeguarding of Covered Defense Information (CDI), which aligns with CUI categories. The clause specifies that the DoD is responsible for determining whether information is Controlled Unclassified Information (CUI) and marking it accordingly before sharing it with contractors. Contractors do not make determinations about what constitutes CUI; they are responsible for safeguarding information once it is received and marked as CUI.

Reference Documents:

DFARS 252.204-7012, Safeguarding Covered Defense Information and Cyber Incident Reporting

CMMC Model v2.0 Overview, December 2021