From Lexicanum:
Cyber-Altered Task units ("CATs") are mechanical drones designed to perform simple tasks, like inorganic servitors. Programmed with a set of very simple instructions, they are occasionally used by the Adeptus Mechanicus and some chapters of the Adeptus Astartes as remote probes and retrieval units. The drones utilise a simple Vox circuit that traverses in a straight line and relay's Picts and other relevant environmental data to the user. When the drone encounters an obstacle they can't negotiate, they rotate ten degrees and continue forward, repeating the process until they can move without impediment. As of M42, CATs had been in service with both bodies for several centuries, or even millennia, although the Mechanicum and the Astartes prefer the versatility and expertise of a servitor or a Scout, respectively. Nevertheless, constructing CATs is a popular hobby for Mechanicus Adepts and Techmarines, who claim that the activity is a useful meditative exercise, and CATs are frequently underfoot at any shrine to the Omnissiah.
In fact, some suspicious members of the Inquisition have observed that their creators seem to treat their CATs almost like pets, though the Mechanicum claims to be above such emotional attachments.
