Final answer:
The use of MARK in military observation is true, indicating when rounds are called for various reasons such as orientation or target indication.
Step-by-step explanation:
The statement regarding the use of MARK in the context of military observation and engagement is True. When an observer employs the term MARK, it typically signifies an action where they are about to call for artillery, mortar, or close air support rounds. The reasons for doing this include orienting themselves within their zone of observation, indicating targets to ground troops, directing aircraft, or other observers, and to mark the optimal height of burst for illumination rounds. Such procedures are crucial for effective communication and coordination during military operations, ensuring precise target engagement and the safety of friendly forces.