Yesterday evening, The Washington Post broke the blockbuster news that FBI agents who searched former President Trump’s Mar-a-Lago residence on Monday were looking for “nuclear documents,” a phrase that immediately set off alarms inside national security circles. The nation’s nuclear systems and plans are considered among the most sensitive and most narrowly known secrets.

Trump denied the report, calling the “nuclear weapons issue” a “hoax.” But assuming the Post’s reporting is correct, what could such a vague phrase as “nuclear documents” mean, and what could we learn about such a category?

Broadly speaking, the US intelligence and defense communities would possess four different categories of files that might be considered “nuclear documents”: nuclear weapon science and design; other countries’ nuclear plans, including the nuclear systems and command of allied nations (UK, France) and adversaries (Russia, China, North Korea, Iran), as well as countries whose nuclear programs exist in a more gray zone (Israel, India, Pakistan); details on the United States’ own nuclear weapons and deployments; and details on US nuclear command & control procedures, known in Pentagon parlance as NC2.

Each category of these documents would carry with it some unique classification peculiarities. And all of them exist at the so-called Above Top Secret level, because a simple Top Secret clearance on its own isn’t enough to access the files.

Security classification procedures really began only in the 20th century, and were codified during the Cold War into three standard levels of classification: Confidential, Secret, and Top Secret, each carrying with it increased levels of control, storage, and more intensive background checks.

Under US law, Top Secret is specifically used to denote “national security information or material which requires the highest degree of protection” and information where, if disclosed, “could reasonably be expected to cause exceptionally grave damage” to national security. Day-to-day, almost anything interesting that the US intelligence or military does exists at that “Top Secret” level. Many US intelligence and military personnel will joke that “confidential” and “secret” information is rarely much more interesting or informed than reading the day’s newspapers.

The wide tranche of operations and intelligence that technically counts as Top Secret means that nearly all sensitive positions in the US government—including FBI agents, many military personnel, and most intelligence officers and analysts—come with a Top Secret clearance and background check standard. All told, according to research conducted by The Washington Post in the wake of 9/11, nearly a million Americans possess Top Secret clearances.

We’ve known since February that Donald Trump apparently took numerous documents from the White House, including documents classified at the Top Secret level. But the reporting this week has added two new wrinkles, both of which hint that the stuff hidden in Mar-a-Lago was even more sensitive.

That’s because virtually all the truly interesting secrets inside the US government aren’t just Top Secret, but come with additional levels of security clearance and special “need to know” access that restrict them even more tightly.

Nuclear science and design files, for instance, are uniquely classified as “Restricted Data.” These files are historically accessed through what’s known as a Q Clearance, a special background check and access protocol. (And yes, the Q Clearance is the “Q” in QAnon, a reference to that anonymous figure’s supposed clearance inside the US government.)

The Restricted Data designation was created by the Atomic Energy Act at the dawn of the Cold War and is now run by the Department of Energy, which oversees the nation’s nuclear weapon stockpiles and development. As nuclear historian Alex Wellerstein explained on Twitter today, the goal was to build a classification outside of the defense establishment that would allow scientific knowledge more flexibility than simply military applications.

“TS/RD” files are what’s known as “born classified,” in that, unlike other classified intelligence or scientific work, they are presumed to be highly classified from the moment of their creation. Effectively, rather than opting into classification, nuclear design and science have to opt out.

Meanwhile, NC2 documents—think documents relating to how the presidential nuclear football operates or how nuclear launch procedures would unfold—have historically had their own classification known as Extremely Sensitive Information (ESI), which again requires special access rights.

Some of the reporting around the Mar-a-Lago search, by ABC News’s Jonathan Karl and others, says that the FBI raid also pertained to what are known as Special Access Programs (SAPs), another unique classification category that usually deals with the most sensitive covert operations and technical capabilities of intelligence and defense systems. (The intelligence community has its own equivalent of the military’s SAPs, which are known as CAPs, or Controlled Access Programs.)

SAPs require someone to be “read into” the program specifically—meaning, they need to have a specific “need to know,” and the documents are carefully tracked to see who has read them and where they’re stored. Usually, individuals are “read into” an SAP in what amounts to a mini-ceremony of sorts, one that involves meeting with a specially cleared security officer and signing a specific nondisclosure agreement for that SAP. Over the course of an official’s career, the SAPs that they’re allowed access to are carefully tracked.

Beyond SAPs, which focus on capabilities, there’s another category of classified information known as SCI, “Sensitive Compartmented Information.” This designation is usually used for protecting what intelligence officials call “sources and methods.” Those could include the identity of a highly placed asset in a foreign government, for instance, or how the NSA has managed to technically penetrate a foreign military’s communication networks. According to Newsweek’s William Arkin, at least some of the documents sought in the FBI search related to “sources and methods.” And The Wall Street Journal reported this afternoon that a list of items removed from Mar-a-Lago includes “various classified/TS/SCI documents.”

SAPs and SCI are known by their own codenames. For example, the long-standing classification for our satellite reconnaissance was TALENT KEYHOLE, so documents protected by it were labeled “TS/SCI TALENT KEYHOLE.” (FBI Director Christopher Wray, who presumably was part of the team that signed off on this week’s Mar-a-Lago search, was a bit player in the Bush administration showdown over one of the best known and most infamous recent SAPs, STELLAR WIND, an NSA wiretapping program created after 9/11.)

Interestingly, for the purposes of the Mar-a-Lago search, SAPs can also protect nuclear research and development as well as the highly secret and protected presidential and military NC2 communication systems, which are known by their own special clearance, YANKEE WHITE.

There are additional levels of document classification restriction the US government uses to show what can be shared with whom: ORCON, which means Originator Controlled, prohibits information from being shared outside of the department or agency where that document was created; NOFORN prohibits information from being shared with any foreign officials; and REL TO FVEY means that the information can be released to countries and officials that are part of the Five Eyes intelligence alliance along with the US: the UK, Canada, New Zealand, and Australia.

Almost regardless of the specifics, any of these “nuclear” categories—SCI, SAP, ESI, RD—denote and protect the most sensitive documents in the entire US government, and penalties for even an inadvertent security breach can be harsh.

Classified documents—and even just conversations about classified information—are never supposed to leave the special reading and conference rooms designed by the US government, known as SCIFs, or Sensitive Compartmented Information Facilities, which are sealed, windowless, specially built, and shielded to be impenetrable to electronic eavesdropping. (The US government even has special Airstream trailers modified to be portable SCIFs for Defense Department VIPs that travel aboard military cargo planes. And when high-level officials like the president travel, security officials build portable SCIFs inside hotel rooms.)

The Justice Department regularly prosecutes those who mishandle or incorrectly take classified documents out of such secure facilities.