The UEF T2 Engineering Station: The Kennel, the UEF T3 Engineering Station: The Kennel and the UEF T1 Engineering Drone: C-D2 "Rover-2" are a set of UEF units. They are engineers.
While by default the drone only roam a limited distance from their Kennel, they can be assigned to a structure, or preferably, an engineer in which case they follow, and assist, the engineer at any distance from their base. You can also manually set drones to reclaim dead units or rocks (mass) or plants (energy) far outside their default range.
Since a Rover's build rate is even higher than a T3 engineer, building multiple Kennels (especially upgraded ones, which contain 2 Rovers) will put a considerable strain on your economy if you use them carelessly, but will massively improve your building efficiency.
Being, for all intents and purposes, flying engineers, Rovers are capable of flying out and either capturing or reclaiming enemy land units or structures with no fear of enemy fire, since most land units cannot attack air units. Their high build rate makes them lethally effective at this. However, even a single unit capable of attacking air units will stop this trick dead. Also, Rovers are air units they can be attacked by strafing interceptors, fighter-bombers and ASFs.