What does the CST condition indicate about an aircraft?

The CST condition stands for "Coast Status," which indicates that an aircraft is not actively being tracked by radar or other computer systems. When an aircraft is in this status, it typically means that it has transitioned out of an actively monitored phase of flight, often due to either a lack of signal or when it is operating outside the usual range of monitoring systems. This situation can arise in areas where radar coverage is limited, leading to a gap in the tracking data.

The other options do not accurately reflect the implications of the CST designation. For instance, low altitude or being on final descent pertain to specific flight phases that would still be actively monitored rather than indicating a loss of tracking. Similarly, being diverted to another airport is a specific operational status that doesn’t relate to whether the aircraft is being tracked. Thus, the correct understanding of the CST condition is essential for interpreting a pilot's or controller's situational awareness when dealing with aircraft not actively monitored.