Given today’s bombshell report by the Washington Post claiming President Donald Trump revealed classified information relating to the Islamic State to senior Russian officials visiting the White House, a brief explainer on levels of classification may be helpful to those unfamiliar with them. According to the press reporting, one U.S. official familiar with the matter described the information Trump shared as “code-word information,” and that a U.S. partner provided it. Bottom line, if that reporting is accurate, this is a major lapse with real national security and diplomatic implications.

The basic classification levels, in ascending order, are CONFIDENTIAL, SECRET, and TOP SECRET. These levels each have a corresponding definition, and can be assigned to information by most federal agencies. For instance, Top Secret means “information, the unauthorized disclosure of which reasonably could be expected to cause exceptionally grave damage to the national security.”

Alongside these classifications, intelligence agencies (e.g., CIA, NSA) put intelligence-based information into “compartments.”   To access such intelligence information, one must be approved generally for “sensitive compartmented information” (called “SCI”), and for the specific compartment. More sensitive information can be further put into “subcompartments,” for which an even more limited group has access. This is often because of the highly sensitive sources and methods used to obtain it.

The baseline, high-level security clearance many national security officials receive is “TS/SCI.” This means they are cleared to see Top Secret information, and are eligible to be “read in” (i.e., granted access) to SCI compartments and subcompartments. The number and identity of people who are read into specific compartments and subcompartments is closely monitored by the intelligence agency administering it.  

Overlying all of this is the concept of “need to know” – that even someone cleared at the right level and read in to the right compartments should only be granted access to particular information if they have a valid need to know it.

“Codeword” information—like what is referred to in the Washington Post report (and since been confirmed by Buzzfeed, the New York Times and Reuters)—refers generally to this highly sensitive, compartmentalized (and sub-compartmentalized) information. The term derives from the fact that a name (i.e., codeword) is assigned to each compartment or subcompartment. In some cases, these names themselves are classified.

In addition, the US receives information from foreign intelligence services, which can themselves impose additional restrictions on sharing based on their own sensitivities and requirements. Generally speaking, if the United States receives intelligence from partner government A, it would not share that information with partner government B unless A had given permission.

The Washington Post story claims that the information Trump shared with the Russians was both codeword and from a partner government. While the facts are still unfolding, this would – if true – represent a serious failure to protect highly sensitive sources and methods. (In his carefully worded statement, General McMaster emphasized that “at no time were any intelligence sources or methods discussed” — but, even so, discussing the existence or details of a particular plot can, in itself, help reveal the existence of the sources and methods from which it was obtained.) In addition, it would jeopardize not only our intelligence relationship with the partner government in question but also with the other governments with whom we share.

To be sure, President Trump has not exposed himself to legal jeopardy – the classification system emanates from the president via Executive Order, and it is within his authority to deviate from it. Still, had another U.S. official revealed information as sensitive as what has been reported, their security clearance would likely be revoked, and, depending on the circumstances, they could face other criminal or administrative consequences. For the president, the consequences – if any – may be political. For our national security, and our intelligence-sharing relationships, the consequences could be worse.

[Editor’s Note: This article was originally published at 9:59PM (ET) on May 15, 2017]