Which element of the refinement and surveillance transmission is mandatory?

The element of the refinement and surveillance transmission that is mandatory is the "End of Mission." This component serves a critical function in the Call for Fire process as it signifies the conclusion of the requested fire mission. By explicitly stating that the mission has ended, it allows all parties involved (including observers, fire direction centers, and forward observers) to know that no further fire support is required for that particular task, ensuring clarity and preventing any potential confusion about ongoing operations.

When the End of Mission is communicated, it also provides an opportunity for reassessment of the battlefield situation by observers and fire support personnel, allowing them to realign their focus on new or ongoing tasks. This clarity is essential for effective communication and coordination in military operations. Other elements like target coordinates, surveillance, and new target range are also important but do not carry the same mandatory status in the context of ending a fire mission.