“This is when the first bombs will definitely drop,” Defense Secretary Pete Hegseth texted a group of senior Trump administration officials, telling them the precise time of military flights that would attack Houthi locations in Yemen. That message, along with many others, was accidentally shared in real time with the editor in chief of The Atlantic.

To many of the people who worked in the classified world of military and intelligence operations, you don’t need a fancy red folder or special government markings to know the plans for an upcoming attack are highly classified. Senior administration officials, however, are staking their reputations on the often bureaucratic nature of classified information.

Here is what to know about how classification of information works.

Is the information classified?

The administration’s defense has rested on whether the details of the messages counts as classified information. Administration officials have noted that it is up to the Defense Department to decide which details of its own work are classified. And since the head of that department, Mr. Hegseth, has declared the information not classified, it therefore is not, they have contended.

But Mick Mulroy, a former Pentagon official in the first Trump administration, said the launch times posted were likely taken from a document outlining the real time battle sequence of the operation against the Houthis. “It is highly classified and protected,” Mr. Mulroy said. “Disclosure would compromise the operation and put lives at risk. Next to nuclear and covert operations, this information is the most protected.”

The administration has vigorously resisted that assessment, pointing to the Defense Department’s role in determining what material would be classified.

“There was no classified information as I understand it,” President Trump said on Tuesday.