A launch sequencer is a critical component of a projectile launching system.
Launch sequencers were the last components necessary to bring the plasma torpedoes online at the Romulan hospital complex on Derna in early 2375. Thus, Colonel Kira Nerys enacted a blockade to keep the warbirds carrying the sequencers from reaching Derna. (DS9: "Shadows and Symbols")
During the last moments of the Silver Blood USS Voyager in 2375, the launch sequencer misfired, preventing the crew from launching a probe containing their logs. (VOY: "Course: Oblivion")
Later that year, a command sensor at the Druoda Strategic Command Matrix malfunctioned, activating a series of launch sequencers. Most of them were shut down, but 34 series 5 long-range tactical armor units were fired towards Salina Prime. (VOY: "Warhead")