32 C.F.R. § 2001.26
(a) Marking information exempted from automatic declassification at 25 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
(b) Marking information exempted from automatic declassification at 50 years. Records exempted from automatic declassification at 50 years shall be automatically declassified on December 31 of a year that is no more than 75 years from the date of origin unless an agency head, within five years of that date, proposes to exempt specific information from declassification at 75 years and the proposal is formally approved by the Panel.
(c) Marking information exempted from automatic declassification at 75 years. Records exempted from automatic declassification at 75 years shall be automatically declassified on December 31 of the year that has been formally approved by the Panel.