The latest guidance is found in Executive Order 13526 (12/29/2009). This updates a series of executive orders issued by previous Presidents. Additional guidance is found in the Code of Federal Regulations (32 CFR 2001).
EO 13526:
http://www.whitehouse.gov/the-press-office/executive-order-classified-national-security-information32 CFR 2001:
http://www.archives.gov/isoo/policy-documents/eo-12958-implementing-directive.htmlInformation may be classified if "its unauthorized disclosure could reasonably be expected to cause identifiable or describable damage to the national security" if it falls into one of the following categories: (a) military plans, weapons systems, or operations; (b) foreign government information; (c) intelligence activities (including covert action), intelligence sources or methods, or cryptology; (d) foreign relations or foreign activities of the United States, including confidential sources; (e) scientific, technological, or economic matters relating to the national security; (f) United States Government programs for safeguarding nuclear materials or facilities; (g) vulnerabilities or capabilities of systems, installations, infrastructures, projects, plans, or protection services relating to the national security; or (h) the development, production, or use of weapons of mass destruction. All classified documents are supposed to cite one of these reasons. Because they are found in Section 1.4 of EO 13526, the reason is often written in the pattern of "1.4 (a)".
Diplomatic cables and other documents that WikiLeaks revealed could be classified under reasons "b" (foreign government information) and "d" (foreign relations or foreign activities of the United States, including confidential sources).
Section 1.7 of EO 13526 lists prohibitions and limitations for classifying information. In general, information cannot be classified to (1) conceal violations of law, inefficiency, or administrative error; (2) prevent embarrassment to a person, organization, or agency; (3) restrain competition; or (4) prevent or delay the release of information that does not require protection in the interest of the national security.
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