(coexisting on another dimensional plane)
In the mirror universe, Starfleet Command, or Imperial Command, was the primary command post of Starfleet on Earth. It served the Terran Empire. It ran daily communiqués to Starfleet starships. (TOS: "Mirror, Mirror")
In 2155, Commander Jonathan Archer claimed to have received a private communiqué from Starfleet Command when he took over the command of the ISS Enterprise (NX-01). (ENT: "In a Mirror, Darkly")
Archer blamed the rebellion on "criminals... sitting in their comfortable offices back at Starfleet Command." (ENT: "In a Mirror, Darkly, Part II")
In 2255, Captain Sylvia Tilly sent a message to the Imperial Command saying that Emperor Philippa Georgiou had arrived safely on the ISS Discovery. (DIS: "Terra Firma, Part 1")
In 2267, Captain James T. Kirk of the primary universe (impersonating his mirror universe counterpart) defied orders from Starfleet Command by delaying the destruction of the Halkans. Starfleet Command then ordered Spock to wait one day to see if Kirk would follow his orders and then to kill him if he did not do so, becoming the new captain of the ISS Enterprise (NCC-1701). (TOS: "Mirror, Mirror")