Final answer:
An extra mast light is added to a vessel when its length exceeds a certain threshold for visibility and safety purposes.
Step-by-step explanation:
In vessels, an extra mast light is added when the length of the vessel exceeds a certain threshold. This is done to ensure better visibility and safety at sea. The specific length at which an extra mast light is required can vary depending on the country and regulations. For example, in the United States, vessels over 50 meters in length are required to have an additional mast light.