What does "say S21523" refer to in air traffic control?

The phrase "say S21523" in air traffic control refers to a specific flight designation. In this context, "S21523" is a unique identifier assigned to a particular flight, allowing air traffic controllers to easily track and communicate with that flight. Flight designations are crucial for maintaining organized and efficient communication within air traffic management, as they help controllers and pilots identify which aircraft is being referenced and ensure clarity during operational communications.

The other choices do not accurately represent the meaning of "S21523." Altitude assignments are typically communicated through different phrasing, while flight information services and position reports involve different types of information and format. Since "S21523" is specifically naming a flight, it distinctly identifies it from these other concepts.