What term describes a fixed signal used to govern the approach to another signal, identified by a "D" marker?

The term that describes a fixed signal used to govern the approach to another signal, identified by a "D" marker, is a distant signal. Distant signals are strategically placed to provide advance warning to train crews about the status of the upcoming signals, allowing them to prepare appropriately. This enhances safety by giving operators time to respond to signal changes that may require them to slow down or stop. The use of the "D" marker specifically indicates that this signal is not controlling the track directly in front of the train but is instead indicating a condition related to a signal that is further ahead. This signaling system helps maintain orderly train movement and improves communication of track conditions.