Former Secretary of State Hillary Rodham Clinton says she never sent classified information through her private email account while she served as America's top diplomat, a claim that has been met with skepticism.

While the State Department is undertaking a review of the emails she turned over to the department, it remains unclear exactly what she included in emails sent from a personal account. In general, however, here is a look at how secretaries of state and their top aides have generally handled electronic communications involving :

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CLASSIFIED INFORMATION

While it may seem unusual, secretaries of state traditionally have rarely dealt personally with classified information in electronic form, according to current and former employees. These employees' knowledge of Clinton's practices is limited, in as much as they have not reviewed her emails and cannot speak to whether she followed standard procedure.

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MAHOGANY ROW AND THE LINE

In Washington, the office of the secretary of state is located on the 7th floor of the department's headquarters along a corridor known as "Mahogany Row." Visitors are required to surrender their cellphones when entering the area, which is lined not only with the secretary's office suite but the offices of other senior department officials and aides. The job of some of these staffers is to receive and send correspondence related to the secretary, sort of a human firewall. Nearly all classified communication to and from the secretary passes through the hands and computers of these staffers, known collectively as "the line."

According to current and past employees, those on "the line" have generally made hard copies of classified documents requiring the secretary's comment, response or signature and hand-delivered them to his or her office for action. Those staffers are also responsible for the ultimate transmission and disposition of the documents once the secretary has responded. Current and past employees say even a secretary of state's references to classified information in are often not sent directly by the secretary but through these aides or other more senior officials on a government .

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ON THE ROAD

When a secretary of state is traveling, the same system is employed on a smaller scale with members of "the line" aboard the secretary's plane and at hotels. On the plane, the secretary's compartment and staff area are designated secure areas where classified conversations and correspondence may be conducted. Those without clearances, notably the traveling press corps, are not permitted in those areas, which constitute the front two-thirds of the converted Boeing 757. In hotels, the State Department often blocks off entire floors or sections of floors as secure areas where classified computers and documents are handled.

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SENSITIVE, BUT UNCLASSIFIED

Unique among federal agencies, the State Department has a designation known as "sensitive, but unclassified," which refers to information that does not meet the standard for secrecy but should nevertheless be handled with extreme care. (Other agencies use a similar formulation "for official use only.") For a secretary of state, that can be particularly true, since much of his or her job involves dealing with other countries, which have their own sensitivities. It's not clear whether any of Clinton's emails from her personal account included language that could embarrass the United States or other governments, like comments from some diplomats that came to light in the WikiLeaks scandal. Since 2005, State Department policy outlined in the Foreign Affairs Manual has been that "sensitive, but unclassified" information should not be included in private emails. However, officials stress that the code of conduct spelled out in the manual is a guideline and not a law or regulation.

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HOW DID CLINTON HANDLE SUCH INFORMATION?

It's still not clear. Her insistence that none of the emails from her personal account—based on a server at her home—contained classified information has been met with skepticism. Critics say the sheer volume of her self-reported work email (30,490 messages on 55,000 printed pages) combined with the sheer volume of classified information available to her as secretary of state make such a claim dubious.

Only a thorough review of the documents—a process the State Department says will take at least several months—will be able to determine if Clinton included any classified material, even perhaps inadvertently, in the emails she gave the department. The review will not include the 31,830 messages that Clinton's office identified as "private, personal records" and did not provide.

Once the emails are reviewed, the State Department says it will post them online for the public to see. But the public will not see every message in its entirety, even if none of them contain classified information. The department will redact the documents on the basis of Freedom of Information Act criteria, which means that unclassified content deemed to be sensitive to national security, personal privacy and trade secrets, among other areas, will be blacked out with an explanation for the redaction.

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INSPECTOR GENERAL'S REPORT

The State Department inspector general's office, in a report out Wednesday, said that in 2011, when Clinton was of state, department employees wrote more than 1 billion emails but only marked 61,156 for the public record. There's no way to know from the figures how many should have been designated as public records. Even fewer, 41,749, were marked for public records in 2013, the year Clinton left the department. One of the issues with Clinton's personal account was that none of the emails she sent during her tenure were to be saved for the public record until she turned over some of them.

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