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

An "X" is employed on a flight progress strip specifically to delete unwanted altitude information. This marking indicates that the specific altitude information is no longer relevant or should be disregarded for the purposes of flight management and coordination. By utilizing an "X," controllers effectively communicate that the previously recorded altitude should not be considered, ensuring that all relevant personnel have clear and updated information regarding the aircraft's flight status.

In air traffic control, maintaining accurate and current flight information is crucial for safe operations. Removing outdated or incorrect altitude data helps prevent misunderstandings or potential safety risks.

The other types of information listed typically remain vital for ongoing flight management. Flight numbers and pilot identification are critical for recognizing and tracking specific flights and their crews, while destination information is essential for routing and planning flight landings.