What type of instructions should not be issued that depend on the movement of an arrival aircraft?

Conditional instructions should not be issued that depend on the movement of an arrival aircraft because they rely on specific circumstances that may not be guaranteed. Conditional instructions often depend on whether a certain event occurs, such as the proper spacing of aircraft or a specific operational situation. Since the movement of arrival aircraft can change rapidly and unpredictably, giving instructions that rely on their movement can lead to confusion or safety risks.

Unconditional instructions, on the other hand, provide clear, straightforward actions that must be taken regardless of other aircraft movements. Advisory instructions are recommendations and do not carry the weight of directive commands, making them different in purpose. Priority instructions focus on urgent matters overriding standard operations, which also do not hinge on the status of an arrival aircraft. Therefore, issuing conditional instructions in such scenarios can create complications in air traffic management and should be avoided to ensure safety and clarity.