What must be done when issuing alternate routing instructions?

When issuing alternate routing instructions, it is essential to avoid issuing clearances as originally filed because the alternate routing indicates a deviation from the initially planned flight path. This can occur due to various factors such as air traffic congestion, weather conditions, or operational requirements. By not issuing the clearance as filed, controllers take into consideration the new routing that has been provided, ensuring the aircraft is directed along the safest and most efficient path given the current situation. This action allows for better management of air traffic and adherence to safety protocols, as the revised instructions help prevent conflicts and maintain orderly traffic flows.

The other options may involve important procedures, such as ensuring clarity on intended altitude or notifying controllers in the vicinity, but they do not directly address the fundamental need to avoid issuing clearances that do not reflect the new routing instructions. Acknowledging receipt of the alternate instructions can be relevant in communication but again does not tackle the primary requirement when alternate routing is necessary.