Automated Personnel Units were sentient humanoid androids first built 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. (VOY episode: "Prototype")
In 2372, the Federation starship USS Voyager discovered an inactive Pralor unit, which chief engineer B'Elanna Torres was able to repair. When Voyager returned Automated Personnel Unit 3947 to his Pralor battleship, the APU kidnapped Torres. She was forced to construct a new chromodynamic power module to enable the Pralor to procreate. Lieutenant Torres succeeded in building a viable prototype but was horrified when 3947 revealed the APU's responsibility for exterminating their Pralor and Cravic creators. Torres destroyed the prototype and was rescued by Voyager when the Pralor were distracted by an attacking Cravic APU battleship. (VOY episode: "Prototype")
In 2409, both Starfleet and the Klingon Defense Force added Automated Personnel Units to their ranks. These served as security officers on starships and were considered sentient lifeforms. (ST video game: Star Trek Online)
Automated Personnel Units were androids powered by a CPM. Their exterior could be modified to reflect faction affiliation. The first APUs, designed by the Delta Quadrant species Pralor and Cravic, were identical except for color. Pralor APUs were sheathed in silver while Cravic APUs were gold. (VOY episode: "Prototype")
Starfleet APUs were based on the same technical principle but took design cues from Soong-type androids. Their look was based on Lal's protoform. The APUs of the Klingon Empire resembled no known species. They were green-skinned and marked by artificial cranial ridges. A KDF APU would activate Termination Protocol 3947 if threatened with dismatling. (ST video game: Star Trek Online)
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)
- Automated Personnel Unit article at Memory Alpha, the wiki for canon Star Trek.
- Automated_Personnel_Unit_(Fed) article at The Star Trek Online Wiki.