What type of light is used primarily for vessel signaling during the day?

The side light is primarily used for vessel signaling during the day as it allows other vessels to identify both the type and direction of a vessel's movement. Side lights, which are red and green, indicate port (left) and starboard (right) sides of a vessel respectively. These lights are crucial for visibility and safety when vessels are navigating in close proximity, especially in conditions where other ambient lighting might be limited.

During daylight, while some vessels may still use their masthead lights for various purposes, side lights are a clear indication to surrounding vessels of your position and heading, which is essential for maintaining safe navigation. The other types of lights mentioned serve different purposes or are meant for visibility at night rather than in daylight conditions.