FOUO (For Official Use Only) in the United States has a very specific meaning and purpose. Although the designation is widely used in both federal and state agencies, in an information security context the marking is meant to prevent the disclosure of the information under the Freedom of Information Act (FOIA).
There are nine specific exemptions and the first exemption (Exemption 1) is for classified documents, meaning documents which have a National Security Classification issued by a Original Classification Authority. For those Original Classification Authority designating a document or other information as FOUO generally protects sensitive but unclassified information.
Exemption 2: "related solely to the internal personnel rules and practices of an agency."
Meant to prevent excessive requests about "matter in which the public could not reasonably be expected to have an interest." Ex: sand types used in paper weights.
Exemption 3: Laws which specifically exempt a type or category of information from FOIA.
Exception 4: "trade secrets and commercial or financial information obtained from a person and privileged or confidential." This is generally information originating from a commercial or individual source which has value to that source.
Ex: A secret family recipe for Tiramisu which was recorded while collecting evidence against a mob boss.
Exemption 5: "inter-agency or intra-agency memoranda or letters which would not be available by law to a party other than an agency in litigation with the agency."
Ex: No requests of performance evaluations for 'under-performing' managers.
Exemption 6: "personnel and medical files and similar files the disclosure of which would constitute a clearly unwarranted invasion of personal privacy."
Ex: The bedwetting habits of FBI informants.
(A) could reasonably be expected to interfere with enforcement proceedings
(B) would deprive a person of a right to a fair trial or an impartial adjudication
(C) could reasonably be expected to constitute an unwarranted invasion of personal privacy
(D) could reasonably be expected to disclose the identity of a confidential source
Ex: What type of sheets FBI informants wet.
Exemption 8:"contained in or related to examination, operating, or condition reports prepared by, on behalf of, or for the use of an agency responsible for the regulation or supervision of financial institutions."
Ex: Why Lehman Brothers was too big to fail.
Exemption 9:"geological and geophysical information and data, including maps, concerning oil wells."
Ex: Why NASA has so much land in florida.