The prefix code was a numbered code individually assigned to each Federation starship. It was a defensive measure designed to help Starfleet personnel combat enemies who had seized control of a ship of their fleet, and to prevent enemy ships from attempting to seize control of a Starfleet vessel.

When the relevant code was entered into the computer system of another ship, that ship was able to remotely control systems on the other ship. This included weapons and defense systems, which enabled the controlling ship to reduce the seized ship's fighting capabilities in order to permit effective attacks. However, a knowledgeable enemy could take the precaution of changing the code.