There is a lot of scuttlebutt surrounding the marking of CUI.

If you are a SUB, your PRIME is responsible for identifying CUI and providing instructions on how to mark the documents.

NOTE: Do not assume everything is CUI – remember, you must be at CMMC Leve 3 or NIST 800-53 to hold and manage CUI.

For general instructions on CUI, see https://www.archives.gov/files/cui/documents/20161206-cui-marking-handbook-v1-1-20190524.pdf