According to a December 2013 policy document released by the Office of the Director of National Intelligence: This designation “is used to protect exceptionally fragile and unique IC (intelligence community) clandestine HUMINT operations and methods that are not intended for dissemination outside of the originating agency.” That is the type of information that is most shielded from interception due to the risk to human lives. Since Clinton was one of the top targets for intelligence services, the transmittal of such information over an insecure line would be grossly negligent and potentially criminal.
Fox is also reporting that the “spillage” of classified information is greater than the “several dozen” emails identified in the January 14 letter to Congress, which also acknowledged for the first time, that the Clinton emails contained intelligence beyond Top Secret, also known as Special Access Programs (SAPs).