Final answer:
A towing light is displayed over the stern light on a vessel when it is towing another vessel, serving as a nighttime indication to other ships that towing operations are underway, and if applicable, the length of the tow.
Step-by-step explanation:
The question refers to maritime navigation lights and their usage. In nautical terms, a towing light is a yellow light positioned over the stern light (white light at the stern) when a vessel is towing another vessel, to indicate their status to other ships at night.
According to the International Regulations for Preventing Collisions at Sea (COLREGs), any power-driven vessel when towing shall show, in addition to the sternlight, a yellow towing light in a vertical line above the sternlight. This helps in differentiating them from vessels not engaged in towing operations.
The towing light is mandatory to indicate towing operations are occurring, and it also provides information about the length of the tow, based on the number of additional lights shown alongside the towing light. If the tow is longer than 200 meters, additional lights are required, thus allowing other vessels to determine the operations taking place and to navigate safely around the towing vessels.