What does the presence of a latching circuit in an electrical diagram signify?

The presence of a latching circuit in an electrical diagram signifies that the circuit can maintain its state even after the initial activating condition has been removed. Specifically, once the latching circuit is triggered and switches on, it will remain in that "on" position until a separate action is taken to manually reset it.

This characteristic is particularly important in various industrial applications, as latching circuits can help maintain system states without continuous input. For instance, after an operator presses a button to start a motor or system, the latching circuit keeps the motor running without the need for the operator to hold the button down.

The other options you mentioned do not accurately reflect the function of a latching circuit. A latching circuit is not designed to indicate a temporary power supply or control compressor speed directly; rather, it establishes a persistent condition that can control equipment until a reset occurs. Also, its operation is not limited to only when the system is off, as it actively provides functionality when the system is engaged.