What is a self-locking circuit?

What does self-locking circuit mean? Self-locking circuit diagram

Literally, it is a circuit that locks itself. See the picture:

In the figure above, buttons 1 and KA are normally open and connected in parallel, and button 2 is normally closed and connected in series in the circuit. When button 1 is pressed, the power is turned on, the KA coil is energized, and the contacts are closed. At this time, even if button 1 is released, the current will still flow through the closed KA contacts, and the KA coil will not lose power, unless button 2 is pressed to cut off the entire circuit. This kind of circuit that locks itself is called a self-locking circuit, which is often used for starting motors, etc.

In our air-conditioning control cabinet circuit, KT3's zero return protection function and cooling and heating fault protection are all self-locking.