Which phrase should NOT be used for communicating an altitude change?

The phrase "Climb and maintain to your discretion" is not appropriate for communicating an altitude change because it implies that the pilot has the authority to decide when to climb and to what altitude they wish to ascend, which can create ambiguity and uncertainty in air traffic control communications. Effective communication in aviation relies on clear directives, and allowing discretion can lead to potential safety issues, particularly in busy airspace.

In contrast, the other phrases provide clear and unambiguous instructions. For instance, "Climb to assigned altitude" clearly states that a specific altitude has been given, which the pilot must reach. "Maintain requested altitude" confirms that the pilot should stay at the altitude they have asked for, while "Descent to the specified altitude" directs the pilot to descend to a particular altitude as instructed. These directives help maintain safety and coordination between pilots and air traffic controllers.