[This post has been UPDATED since first published]

Occasionally in the news (and more often in spy fiction) people pass around super-secret documents marked “Eyes Only.” The United Kingdom and Canada use “Eyes Only” to indicate specific countries with whom a document may be shared. “UK Eyes Only,” for example, means that the document is only distributed within the UK and not to other countries. The marking may also have a list of countries, often the “five eyes.” The US has traditionally used other markings like REL TO (release to) or NOFORN (no foreign dissemination) for this purpose. This type of marking is often called a caveat as opposed to being a classification level, compartment, or codeword.

Classified document markings are treated seriously, and government agencies have published explicit definitions of them. US classification guides (for example, this one from the intelligence community) discuss “Eyes Only” exclusively in terms to the UK definition. This does not, however, cover all examples. The only other US definition I’ve found dates from 1974:

c. Eyes Only Messages.-A privacy communication from one named individual to another individual. Delivery of this type of message is restricted to the named addressee or to those personnel (contacts) the addressee has authorized to receive such messages. No further dissemination is permitted unless so directed by addressee or authorized member of his staff.

Defense Intelligence Agency (DIA), Special Operations Branch Office Procedure No. 35-2, January 13, 1974 (quoted from a Senate hearing transcript later that year).