Automated Personnel Units were androids created by the Pralor and the Cravic in the early 23rd century to fight each other. They were powered by chromodynamic power modules. When the Pralor and Cravic agreed to end their war they also agreed to dismantle the Automated Personnel Units, but the units turned on their masters and eradicated them before continuing the war.
In 2372, the USS Voyager discovered an inactive Pralor unit, which B'Elanna Torres was able to activate and create new chromodynamic power modules for. Upon realizing these power modules would be used to give the Pralor units a massive advantage she destroyed her work. (VOY episode: "Prototype")
In an alternate timeline, Torres' experience with the chromodynamic power modules made her perfect for the mission to the Casciron ship which she would hide aboard after disabling the Vostigye vessel, Ryemaran. (Star Trek: Myriad Universes novel: Places of Exile)