Turns out there's a sort of a cheat sheet related to FOIA document exemptions that can aid you in your digging into such records. Emma Best at MuckRock dug up the codes with some advice for how to get the best out of your search. Exemptions are often applied in an arbitrary, sometimes even mystifying manner. If you're ready to play Sherlock you can still infer a lot of information from the exemption codes themselves throughout the course of your hunt.
In other words, being hit with an exemption as you search for some specific information may not entirely be a dead end. Keeping an eye on the exemption codes being used may be somewhat telling if you know what you're looking for.
25X1 is one of the most commonly cited exemptions to keep some information out of the hands of the public. Technically it's the exemption for "sources and methods." Of course the security state has trade secrets they'd like to keep under wrap. Too often the class "25x1" exemption is just slapped on as a pretext for keeping some data under lock and key. Best notes one case where the name of an Eastern Bloc beer brand was redacted under this code. As some justification, if you've ever seen a James Bond film, you'll have to admit that alcohol and it's copious ingestion are certainly employed as material <i>and</i> method for Iam Fleming's fictional hero.
25X2 is "information that would assist in the development, production, or use of weapons of mass destruction." If you find this exemption a little geiger counter in your head should be going off because you're in the presence of something radioactive or equally destructive. Best warns that sometimes WMDs may only be tangentially connected to a file for it to be suppressed using this exemption. Combining this exemption code with terms like ICBMs or other related terms while searching databases may cull the nearly 600,000 results found in the CIA's CREST database which was only digitized as of early 2017.
25X3 demarcates “information that would impair U.S. cryptologic systems or activities.” When you run across this you're in the realm of the NSA, surveillance systems and documents relating to encryption and decryption.
Code 25X4 is “information that would impair the application of state-of-the-art technology within a US weapon system.” If you've run into this code you're hitting on some bleeding edge technology such as the military space program and the Presidentical Directive/NSC Directive
25X5 exemption code is reserved for documents “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.” Like code 25X1 this is a code that Best warns could be dubious as it's used improperly to as in the case of some CIA photographs of Soviet astronomical observatories.
25X6 might be the kind of exemption used to hide the fact that under Obama our surveillance efforts extended to our allies. Tapping German prime minister Angela Merkel's phone could certainly be construed as “information including foreign government information, that would cause serious harm to relations between the U.S. and a foreign government, or to ongoing diplomatic activities of the U.S.” A host of sins can be hid under this code such as CIA psychological warfare tactics.
25X7 is specifically for “information that would impair the current ability of U.S. government officials to protect the President, Vice President, and other protectees for whom protection services, in the interest of national security, are authorized.” Items related to the President's itinerary might turn up here.
25X8 stands for “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.” According to Best some really interesting material related to the CIA's emergency relocation plan is shuttered behind this code.
25X9 pertains to classified information that is protected “a statute, treaty, or international agreement that does not permit the automatic or unilateral declassification of information at 25 years.” Once again, it is used as a flimsy excuse to cover up information.
<quote>This can cover almost anything, including CIA’s desire to transfer certain staff employees to contract employees (a distinctly different status, which, among other things, allows CIA additional deniability).</quote>
Combining the exemption codes can help you to refine your search. In addition to keeping track of national security history and shedding more light on current events with the help of uncovered documents, MuckRock also will assist you in <a href="https://www.muckrock.com/foi/create/">making your own FOIA filings</a>. You can also search through their archives for <a href="https://www.muckrock.com/foi/">filed requests</a>.