What to know about government classification of secrets.

The US government has a formal system of protecting information that, if disclosed, could hurt national security.

The system can apply to documents regarding intelligence activities, foreign relations, military plans and programs for safeguarding nuclear materials, for example. By classifying information, the government restricts who can see the documents and where he or she can see them.

The Justice Department recently removed some classified documents from former President Donald Trump’s Mar-a-Lago residence while executing a search warrant for possible violations of the Espionage Act and other crimes.

Here are key things to know about how the classification system works.

What are the levels of classification?

There are three basic levels of classification, based on the damage that could be done to national security if the information was leaked.

Top Secret

This is the highest level of classification. Information is classified as Top Secret if it “reasonably could be expected to cause exceptionally grave damage to the national security,” according to a 2009 executive order that describes the classification system.

A subset of Top Secret documents known as SCI, or sensitive compartmented information, is reserved for certain information derived from intelligence sources. Access to an SCI document can be even further restricted to a smaller group of people with specific security clearances.

Some of the materials recovered from Trump’s Florida home were marked as Top Secret SCI.

Secret

Information is classified as Secret if the information is deemed to be able to cause “serious damage” to national security if revealed.

Confidential

Confidential is the least sensitive level of classification, applied to information that is reasonably expected to cause “damage” to national security if disclosed.

What might be classified as a Top Secret document?

“The difference between Secret and Top Secret is quite significant,” said Philip Mudd, a CNN counterterrorism analyst and former CIA analyst.

Nuclear codes could be classified as Top Secret, for example, as well as information from an especially sensitive human source.

But there are even more narrow classifications within the Top Secret designation. For example, intercepted communications may be classified as Top Secret SCI.

Another category of sensitive information within either Top Secret or Secret documents is known as an SAP, or special access program. Not everyone with a Top Secret security clearance may be privy to the information in an SAP.

These higher level designations further restrict who can see the information and where it can be viewed.

How is information classified?

Certain people within the government have classification authority. For less sensitive classified material, he or she may make a judgment call about the impact the information could have on national security if disclosed.

But a classification of Top Secret SCI would be automatic, said Mudd.

That may be because the information would reveal a source the United States uses or a certain method of obtaining intelligence information.