What are the requirements regarding the horizontal sectors of sidelights?

The requirement that sidelights must have practical cut-offs specified by international regulations is important because it ensures that the lights effectively indicate a vessel's presence and course while minimizing confusion and misinterpretation, especially in crowded or busy waters. The COLREGs (International Regulations for Preventing Collisions at Sea) provide specific guidance on the angles of visibility for these lights, meaning that they should be constructed and positioned in such a way that they are not visible outside of their defined sector when another vessel is approaching from the side. This cut-off helps achieve clarity in navigation by preventing overlap of visual signals between vessels. Moreover, defined horizontal sectors help prevent any unintended visibility to an oncoming vessel, which could mislead or cause confusion regarding the other vessel's orientation or intentions.

The other options do not align with the regulations. For example, sidelights are crucial for vessels of various sizes and are needed for safety regardless of whether visibility conditions are poor or the vessel is under a certain size. Additionally, height requirements are not specified universally across regulations, thus making the statement regarding a specific height invalid in this context.