When should it be applied to contractors handling CUI?

As in which types of CUI Specified require adherence to one or more requirement from 172?

I can’t find any, the NIST people can’t answer the question… to me 172 seems to be a useless document for CUI.

The question could also be stated as: “for which types of CUI handling does CMMC set Level 3? (Or level 5 in version 1)