From a 1971 "Official Global Autovon Telephone Directory" which has no classification (not even "For Official Use Only"):

Transmission Pre-emption

Has precedence over any other telephonic call of lower precedence.
Pre-empts lower precedence calls.  May be pre-empted by the
application of the FLASH OVERRIDE capability available to: (1) The
President of the United States, Secretary of Defense and Joint Chiefs
of Staff; (2) Commanders of unified and specified commands when
declaring either Defense Condition One or Defense Emergency; (3)
CINCNORAD when declaring either Defense Condition One or Air Defense
Emergency and other national authorities as the President may
authorize.

Application

Flash precedence is reserved generally for telephone calls pertaining
to: (1) Command and control of military forces essential to defense
and retaliation; (2) Critical intelligence essential to national
survival; (3) Conduct of diplomatic negotiations critical to the
arresting or limiting of hostilities; (4) Dissemination of critical
civil alert information essential to national survival; (5) Continuity
of Federal Government functions essential to national survival; (6)
Fulfillment of critical United States internal security functions
essential to national survival; (7) Catastrophic events of national or
international significance.

Transmission Pre-emption

Has precedence over any other telephonic call of lower precedence.
Pre-empts lower precedence calls.

Application

Immediate precedence is reserved generally for telephone calls
pertaining to: (1) Situations which gravely affect the security of
national and allied f6rces; (2) Reconstitution of forces in a
post-attack period; (3) Intelligence essential to national security;
(4) Conduct of diplomatic negotiations to reduce or limit the threat
of war; (5) Implementation of Federal Government actions essential to
national survival; (6) Situations which gravely affect the internal
security of the United States; (7) Civil Defense actions concerning
direction of our population and their survival; (8) Disasters or
events of extensive seriousness having an immediate and detrimental
effect on the welfare of the population; (9) Vital information
having an immediate effect on aircraft, spacecraft, or missile
operations.

Transmission Pre-Emption

Has precedence over any other telephonic call of lower precedence.
Pre-empts lower precedence calls.

Application

Priority precedence is reserved generally for telephone calls
requiring expeditious action by called parties and/or furnishing
essential information for the conduct of government operations.

Transmission Pre-emption

Has no precedence over any other call and is handled sequentially as
placed by the calling party.

No pre-empt.

Application

Routine precedence designation applies to those official government
communications which require rapid transmission by telephonic means
but do not require preferential handling.