Which of the following is a required element for subsequent fire commands?

In the context of subsequent fire commands, the required element is typically the AET, or Adjusted Engagement Target. This element is crucial for coordinating and accurately executing the fire mission, as it provides a clear target or reference point for the gunners to focus their fire. Including the AET ensures that all personnel involved in the operation are on the same page regarding the engagement area, which ultimately enhances the effectiveness and safety of the operation.

The AET helps in making precise adjustments based on the previous engagement's outcome and any changes in the tactical situation. This means that the composition of fire commands can adapt to dynamic conditions on the battlefield, ensuring continuous support and more effective targeting as the situation evolves.

Other elements like assessment, execution, and termination are important parts of the broader fire support process but are not strictly required as elements in subsequent fire commands in the same way that the AET is. These other aspects may vary based on the mission's requirements or operational context, while the AET is a consistent necessity for subsequent commands to maintain clarity and direction in targeting.