32 CFR § 2001.26(a): Automatic declassification exemption markings [32 CFR CHAPTER XX—INFORMATION SECURITY OVERSIGHT OFFICE, NATIONAL ARCHIVES AND RECORDS ADMINISTRATION: 32 CFR PART 2001—CLASSIFIED NATIONAL SECURITY INFORMATION]
(a) Marking information exempted from automatic declassification at 25 years.
(1) When the Panel has approved an agency proposal to exempt permanently valuable information from automatic declassification at 25 years, the “Declassify On” line shall be revised to include the symbol “25X” plus the number(s) that corresponds to the category(ies) in section 3.3(b) of the Order. Except for when the exemption pertains to information that should clearly and demonstrably be expected to reveal the identity of a confidential human source, or a human intelligence source, or key design concepts of weapons of mass destruction, the revised “Declassify On” line shall also include the new date for declassification as approved by the Panel, not to exceed 50 years from the date of origin of the record. Records that contain information, the release of which should clearly and demonstrably be expected to reveal the identity of a confidential human source or a human intelligence source, or key design concepts of weapons of mass destruction, are exempt from automatic declassification at 50 years.
(2) The pertinent exemptions, using the language of section 3.3(b) of the Order, are:
25X1: reveal the identity of a confidential human source, a human intelligence source, a relationship with an intelligence or security service of a foreign government or international organization, or a non-human intelligence source; or impair the effectiveness of an intelligence method currently in use, available for use, or under development.
25X2: reveal information that would assist in the development, production, or use of weapons of mass destruction;
25X3: reveal information that would impair U.S. cryptologic systems or activities;
25X4: reveal information that would impair the application of state-of-the-art technology within a U.S. weapon system;
25X5: reveal formally named or numbered U.S. military war plans that remain in effect, or reveal operational or tactical elements of prior plans that are contained in such active plans;
25X6: reveal information, including foreign government information, that would cause serious harm to relations between the United States and a foreign government, or to ongoing diplomatic activities of the United States;
25X7: reveal information that would impair the current ability of United States Government officials to protect the President, Vice President, and other protectees for whom protection services, in the interest of the national security, are authorized;
25X8: reveal information that would seriously impair current national security emergency preparedness plans or reveal current vulnerabilities of systems, installations, or infrastructures relating to the national security; or
25X9: violate a statute, treaty, or international agreement that does not permit the automatic or unilateral declassification of information at 25 years.
(3) The pertinent portion of the marking would appear as: Declassify On: 25X4, 20501001
(4) Documents should not be marked with a “25X” marking until the agency has been informed that the Panel concurs with the proposed exemption.
(5) Agencies need not apply a “25X” marking to individual documents contained in a file series exempted from automatic declassification under section 3.3(c) of the Order until the individual document is removed from the file and may only apply such a marking as approved by the Panel under section 3.3(j) of the Order.
(6) Information containing foreign government information will be marked with a date in the “Declassify On” line that is no more than 25 years from the date of the document unless the originating agency has applied for and received Panel approval to exempt foreign government information from declassification at 25 years. Upon receipt of Panel approval, the agency may use either the 25X6 or 25X9 exemption markings, as appropriate, in the “Declassify On” followed by a date that has also been approved by the Panel. An example might appear as: 25X6, 20600129, or 25X9, 20600627. The marking “subject to treaty or international agreement” is not to be used at any time.