What additional line of the CAS 9-line is mandatory readback when working with NATO forces?

The correct answer is Line 8, which is specifically required for readback when coordinating Close Air Support (CAS) with NATO forces. Line 8 provides crucial information regarding the "Target Mark Type," which clarifies how the target will be defined or marked for the aircraft conducting the CAS mission. In NATO operations, standardization is key for effective communication, particularly in joint operations where different national forces may interact. Ensuring that Line 8 is read back helps to avoid any misunderstandings and enhances situational awareness among all participating units, thereby ensuring a higher level of safety and coordination during missions.

The other lines, while still important in the overall CAS brief, do not carry the same mandatory readback requirement in NATO procedures. This highlights the unique nuances that exist within international military operations and emphasizes the focus on clear, unambiguous communication in a joint environment.