My source is
DOD regulation 5400.7 paragraphs C184.108.40.206.5 and C220.127.116.11.1.3.1 These paragraphs have to do with the freedom of information act (FOIA). It is a method of denying information considered exempt by the act.
The first time was used was a response from the CIA to a FOIA request. It was dealing with the Glomar Explorer, a ship supposedly built to recover a sunken russian sub. That is how it got its name.