A vessel that is not under command and underway but not making way would display what?

A vessel that is not under command and underway but not making way is required to display two all-round red lights in a vertical line. This specific signal is used to indicate to other vessels that the vessel is unable to maneuver as required due to some overriding issue, such as mechanical failure or other emergencies.

The two all-round red lights serve as a clear and conspicuous warning to approaching vessels, signifying that the vessel cannot comply with the navigation rules and may present a danger. Visibility of this signal is crucial to ensure the safety of other vessels in the vicinity, allowing them to take necessary precautions when navigating near the vessel.

Other signals, such as one all-round white light or other combinations of colored lights, do not convey the same situation of being not under command. The intention behind the display of two all-round red lights is specifically linked to the inability of the vessel to navigate effectively, distinguishing it from other operational statuses of vessels.