Which of the following statements is true about the use of DCD in mainline operations?

The correct statement about the use of a Descent Control Device (DCD) in mainline operations is that it should be in the maximum friction position. This is crucial because positioning the DCD in maximum friction increases the resistance to descent, which helps in controlling the speed of lowering a load or a rescuer. Proper friction settings are essential for maintaining safety and ensuring that any descent is managed effectively without uncontrolled rapid movements.

This is important in rescue scenarios where maintaining control over the descent is critical to the safety of the person being lowered as well as the rescuer. If the DCD is not set for maximum friction during practice or operational use, it could lead to unsafe situations, especially in challenging environments or during critical rescue missions.