An "X" on a flight progress strip is used to delete what type of information?

The use of an "X" on a flight progress strip specifically indicates an instruction to delete unwanted altitude information. In the context of air traffic control, altitude assignments can change frequently due to various operational factors, and the "X" serves as a clear visual cue to remove specific altitude-related entries that are no longer applicable.

This allows controllers to maintain accurate and current information on flight progress strips, ensuring that they have the most relevant data at their fingertips. By eliminating outdated or unnecessary altitude information, controllers can prevent potential misunderstandings or errors in communication related to aircraft altitude assignments, which is crucial for maintaining safe and efficient air traffic management.

While the other options may represent other types of information on a flight progress strip, they are not directly associated with the function of the "X" marking, which uniquely pertains to altering altitude details. Thus, understanding this specific use case helps controllers ensure clarity and precision in flight data handling.