FOUO, for official use only. That’s non-secret but not appropriate for general release information (for some reason). CUI is the updated version of that level of classification.
FOUO - A doctors patient list. That isn’t the business of the public and should not be released. That data does not contain PII, patient health data, and is not protected as a matter of security but it is still protected data.
Classified - Data that is restricted from public release. This is lowest level of protection and is intentionally vague.
Secret - Information that if disclosed can be used to harm people or disrupt government operations by adversaries. This includes information like convoy travel schedules and communications outages. Secret data is typically really boring office information in otherwise more exciting work.
Top Secret - This information, if disclosed, will likely result in embarrassment to the extent that national security or diplomatic relations suffer or that the intentional imminent danger of death or bodily harm occurs to people.
e.g. all Secret is Classified but not all Classified is Secret.
https://www.esd.whs.mil/Portals/54/Documents/DD/issuances/do...