What best defines a supplementary fighting position?

A supplementary fighting position is best defined as a position guarding against unexpected attacks. This definition highlights its purpose in a broader tactical context. Supplementary positions are established to enhance the overall defensive capability of a unit, ensuring that it can respond effectively to threats from various directions, particularly those that may catch the primary defenders off guard. By having these supplementary positions, forces can create a layered defense, allowing them to maintain operational readiness and resilience against surprise assaults.

In contrast, the other choices do not accurately capture the essence of what a supplementary fighting position is meant to accomplish. A primary defense point focuses on a main area of defense, while a position designed for retreat emphasizes withdrawal rather than proactive defense. A location used exclusively for sharpshooters suggests a specialized function that does not encompass the broader defense role that supplementary positions serve. Therefore, the correct choice reflects the inherent purpose of such positions in military operations.