What is indicated by a vessel displaying 2 black balls in a vertical line?

A vessel displaying two black balls in a vertical line indicates that the vessel is at anchor. According to the International Regulations for Preventing Collisions at Sea (COLREGs), a vessel at anchor must display an appropriate day shape indicating its status to other vessels in the vicinity. The use of two black balls signifies anchorage, specifically used during the day to communicate that the vessel is stationary and not maneuvering.

The other options relate to different scenarios: a vessel underway would not display these shapes, as it would need to show side lights and a stern light instead. A vessel not under command would display two black balls in a vertical line at night instead of in the daytime, along with specific lights. A vessel restricted in its ability to maneuver would also show different shapes or lights according to its particular situation. Thus, displaying two black balls correctly informs other vessels that the vessel is at anchor.