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Special Agent in Charge is a rank within the FBI. It is often referred to as S.A.C..

In 1995, Walt Eubanks held this rank. (TXF: "Oubliette")

In 1998, Darius Michaud held this rank. (The X-Files Movie)

In 2008, Dakota Whitney, who was otherwise known as an ASAC, introduced herself to former agents Mulder and Scully as "Special Agent in Charge Whitney." Special Agent in Charge Fossa was another FBI official who held this rank at the same time as Whitney, and the two were known to work together before Whitney's death shortly later that year. (The X-Files: I Want to Believe)

Dakota Whitney's introduction as a Special Agent in Charge and a reference to her as an ASAC (by Special Agent Drummy) imply that the two rank titles are interchangeable.
Although Special Agent in Charge Fossa introduces herself as exactly that, saying her full rank title and surname, during The X-Files: I Want to Believe, the film's end credits refer to the character only as Special Agent in Charge.
In a scene that appears in the script of TXF: "Deep Throat", recurring character Scott Blevins is described as holding the rank of Special Agent in Charge. Blevins makes no appearance in the final version of the episode and no other sources support the idea of him holding this rank or having held it.
Ranks and Titles Within the FBI
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