What must a vessel that is not under command display at night?

A vessel that is not under command must display two red lights in a vertical line at night. This signaling indicates to other vessels that the boat is unable to maneuver or proceed due to circumstances such as being disabled or having mechanical failure. The two red lights serve as a clear warning to other mariners, thus ensuring safety on the water by alerting nearby vessels to exercise caution when in proximity to the incapacitated vessel.

Other signaling options, such as black balls or cylinders, do not conform to the internationally accepted regulations for vessels in distress or unable to navigate. The standard for indicating a vessel not under command is specifically the use of the two red lights to ensure clarity and prevent collision.