WASHINGTON (AP) — Democratic presidential candidate Hillary Rodham Clinton is under scrutiny over whether she sent or received classified information on unsecured email when she was secretary of state. The inspector general of the U.S. intelligence community recently alerted the Justice Department about classified information included improperly on email that went through a home server Clinton used in lieu of the official State Department email system.

The "referral" to the Justice Department laid open an ongoing difference of opinion between the State Department and the nation's intelligence agencies over what material should be classified and what should not. Here's a look at how government information is made secret — or not.

Q: How could classified information be floating about?

A: None of the allegedly secret information in the emails was marked "classified," the IG found. But just because something is not stamped "Top Secret" doesn't mean it isn't. Some of the email, he said, included information derived from intelligence agencies that should have been marked classified and sent over secure channels.

But the question of whether certain information is classified amounts to a judgment call. The State Department may disagree with the IG about whether information in the emails was classified at the time it was sent.

"It's not a precise mathematical calculation," said Steven Aftergood, who writes about secrecy for the Federation of American Scientists. "Plus, the law is permissive, not mandatory. It says that if information meets certain conditions it may be classified, not that it must be."

Whether it can be withheld from the public now is a different question — it is possible for the government to classify material retroactively.

Q: So what constitutes classified information? Is there a definition?

A: Yes, but it's incredibly broad. It's found in Executive Order 13526, which says classified information must fit into the following categories: military plans, weapons systems, or operations; foreign government information and foreign relations; intelligence activities, sources or methods; scientific, technological, or economic matters relating to national security; plans and programs for safeguarding nuclear materials or facilities; vulnerabilities or capabilities of systems, installations, infrastructures, projects, plans, or protection services relating to national security; and almost anything related to weapons of mass destruction.

Q: So who decides what information is classified?

A: A variety of government officials, from the president on down. And while they are not supposed to classify information improperly, it happens every day. So-called "overclassification" is seen as a massive problem, because it's so easy to stamp something secret. There is also a bureaucratic rigidity to the system that defies common sense. Government officials without the proper clearances are under orders not to read classified surveillance documents leaked by Edward Snowden or State Department cables disclosed by Wikileaks, for example, even when the rest of the public can do so. The line becomes further blurred when a government official gets information through open channels that was included in a classified document he or she read.

Q: If Clinton did send or receive classified information over her home email server, what are the implications for her — and for national security?

A: That is difficult to assess without knowing more about the sensitivity of the material. But there is a context to keep in mind. Critics say her private email server was almost certainly vulnerable to foreign intelligence services such as China's or Russia's. However, the State Department's unclassified email system is also believed to have been penetrated by Russian hackers. "It's conceivable that they were more secure on her home server than on the State Department, server which would be ironic," Aftergood said. That would not be the case, though, if she had sent the material via the State Department's classified system, which is considered more secure.

There is also a history of senior government officials who have gotten in legal trouble for mishandling classified information, even when there was no evident damage to national security. The most recent was former CIA director David Petraeus, who pleaded guilty to a misdemeanor after admitting he gave diaries containing highly sensitive secrets to his biographer and girlfriend, Paula Broadwell, though she never published them. In 1996, CIA Director John Deutch resigned after it was discovered that he had stored highly classified documents on a home computer connected to the Internet.