Multiple realities
(covers information from several alternate timelines)

The Chief of Starfleet Security was a Starfleet flag officer who oversaw Starfleet Security. In 2367, Vice Admiral Thomas Henry held this post, followed by Admiral Raner in 2370. (TNG: "The Drumhead", "The Pegasus")

In 2371, Benjamin Sisko took the decision to relieve Odo of his duties regarding security for Starfleet operations- Michael Eddington having been appointed head of Starfleet security matters on the station while Odo remained responsible for the station's Bajoran Militia detachment and civilian population- to the Chief of Starfleet Security herself, but was unable to have it overturned. (DS9: "The Search, Part I")

In an alternate 2372, Admiral Strickler was scheduled to have a meeting with the head of Starfleet Security. Strickler noted that she was a person who "doesn't like to be kept waiting." (VOY: "Non Sequitur")

It is not stated whether these female Chiefs of Starfleet Security were the aforementioned Raner, or another flag officer(s) who replaced her.

Later in 2372, Benjamin Sisko's knowledge of the Founders led to his brief promotion to acting Chief of Starfleet Security on Earth. (DS9: "Homefront")

As of 2399, Commodore Oh was director of Starfleet Security. (PIC: "The End is the Beginning")

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