A control unit is a command core: two processors, an antenna and a frame, put together in an Assembler. It goes into the relays that act as command posts in the Infonet, the network that carries orders to ships: an Uplink is built around one, a Forward HQ around two, and the flagship carries one. The ark and every drive core carry one as well, a gate core takes two, and the battery, torpedo bay and shield generator fittings take one each.
Control units take more copper than any other item of their grade, the number of crafting steps behind an item. Most of that copper is found from the Static outward.