What light requirement is needed for vessels when they are not underway?

When vessels are not underway, they are typically at anchor. The specific light requirement for vessels in this situation is the all-round white light, which is essential for visibility to other boats. This light serves as a signal to indicate that a vessel is anchored and not in motion, helping to prevent collisions, especially in low visibility conditions such as night or fog.

While other colors like red and green lights are used when a vessel is underway to indicate its status and direction, they are not sufficient on their own when the vessel is at anchor. The absence of lights is not a viable option either, as it would create a significant hazard for nearby vessels. The starboard light, which is typically green, is also not applicable when the vessel is anchored because it does not convey the necessary information about the vessel's status. Therefore, the use of an all-round white light is the required standard to ensure safety when the vessel is not moving.