What is the forward surge current of a diode?

What is the forward surge current of a diode? What is the difference in definition between him and the positive rated current? Their essential difference lies in the two words' impact ', so what is their source?

Switching power supplies require diodes to withstand large surge currents without breaking during startup or other transient situations. Of course, these surge currents should be non repetitive or have long intervals.

Usually, the data manual for rectifier diodes defines this surge current, and its testing conditions are often a single waveform surge current, such as a single sine wave or square wave. Its current value can often reach several hundred.