If you bothered to read the source article

Emails sent over the low side sometimes were informal discussions that occurred in addition to more-formal notifications through secure communications, the officials said.

One such exchange came just before Christmas in 2011, when the U.S. ambassador sent a short, cryptic note to his boss indicating a drone strike was planned. That sparked a back-and-forth among Mrs. Clinton’s senior advisers over the next few days, in which it was clear they were having the discussions in part because people were away from their offices for the holiday and didn’t have access to a classified computer, officials said.

The CIA drone campaign, though widely reported in Pakistan, is treated as secret by the U.S. government. Under strict U.S. classification rules, U.S. officials have been barred from discussing strikes publicly and even privately outside of secure communications systems.

The State Department said in January that 22 emails on Mrs. Clinton’s personal server at her home have been judged to contain top-secret information and aren’t being publicly released. Many of them dealt with whether diplomats concurred or not with the CIA drone strikes, congressional and law-enforcement officials said.

Several law-enforcement officials said they don’t expect any criminal charges to be filed as a result of the investigation, although a final review of the evidence will be made only after an expected FBI interview with Mrs. Clinton this summer.

One reason is that government workers at several agencies, including the departments of Defense, Justice and State, have occasionally resorted to the low-side system to give each other notice about sensitive but fast-moving events, according to one law-enforcement official.

(Emphasis mine)