What does a horizontal arrow (→) in space 20 indicate?

A horizontal arrow (→) in space 20 indicates that the aircraft has been cleared to cruise. This symbol is used in air traffic control communications to signify that the aircraft has reached its cruising altitude and is authorized to proceed on its flight path without further altitude adjustments. It represents stable flight conditions where the aircraft is operating efficiently at a predetermined altitude.

The other options relate to different flight conditions or instructions. For instance, a cleared to land would typically be indicated by a different symbol or message that signifies an approach to an airport, while experiencing turbulence would involve advisories to the pilot indicating fluctuations in air currents. A holding pattern requires specific instructions and typically has associated communication or indications that differ from a cruising clearance. Understanding these symbols and their meanings is crucial for effective communication between pilots and air traffic controllers.