For example, if you design a comparator, the input voltage will not be output until 30V. When 30V has an output, the input voltage will drop to 25V and the output will be turned off. But if you want to output again, the input voltage must be 30V. This 5V voltage between 25V and 30V is called return difference voltage.

Because of the existence of return voltage, Schmidt trigger has strong anti-interference ability and is suitable for waveform shaping.