Under that system, the same set of facts can be considered classified in one agency and unclassified in another, depending on their source. Similarly, open source information can be kept classified by the U.S. government for years after it's made public in news reports or leaks.
Even as Clinton has been showing more contrition
on her decision to use a private email server while at the State Department, members of her campaign are hitting back on the notion she violated regulation governing the transmission of classified information and taking aim at what they say is over-classification.
As an example, they point to an email forwarded to Clinton by top aide Huma Abedin that is now at the center of the intelligence community's review of Clinton's emails.
"This email was marked unclassified when it was sent, it remains unclassified now, and large portions of its contents were being publicly reported at the time by multiple outlets," Clinton adviser Karen Finney told CNN. "It is increasingly clear that the focus belongs on untangling the inconsistent and discordant web of practices various government agencies use to determine what and how information is or isn't classified."
The email, originally sent to Abedin by another State Department employee, Timmy Davis, features information about an attack on the Libyan town of Adjabiya by forces loyal to Moammar Gadhafi -- information Davis learned from government sources, but which was also known outside the government.
For instance, a CNN report
published one day before the Davis email was sent quotes a Libyan opposition leader as saying, "There were some 50 regime pickup trucks with machine guns and rocket launchers that attempted to enter Ajdabiya."
This matches what Davis wrote to Abedin and others about Gadhafi forces striking the city's western gate with 50 vehicles.
Nevertheless, Davis specifically attributes the information in his email to non-public sources, including a United States Africa Command assessment and the British government.
Davis also includes British analysis of how recent movement by Gadhafi forces from Sirte to Brega could foretell future attacks on Ajdabiya.
Even when information is publicly available, it can still be classified by U.S. government agencies.
In 2012, the American Civil Liberties Union sued the government for refusing to release classified information that had already been posted online by the group WikiLeaks. The judge in that case ruled in favor of the State Department, re-affirming that classified information remains classified even when it's widely known.
"The government's rules for how information is classified and secured are in place to protect our national security," Judiciary Committee Chairman Chuck Grassley said in a statement. "The agency that classifies a piece of information, whether it be the State Department or someone within the Intelligence Community, is the only agency authorized to declassify that information."
"The material in question was classified by other agencies in the Intelligence community -- not by the State Department," he added. "You don't get to ignore or re-write the rule book just because you don't like the rules."
In the case of the Davis email, there are also broader questions in play about why the State Department didn't think sensitive information regarding the security of U.S. diplomats warranted a higher classification.
Davis writes about plans for then-special envoy Chris Stevens, later killed during the attack on the U.S. consulate in Benghazi, to possibly leave the area if security conditions worsened.
But confidential or not, those plans constitute sensitive information that many critics say should not have been communicated through an unclassified email system.