A current limiting diode is a junction field effect transistor (depletion mode, fully on and turned off with reverse bias on the gate) that has its gate connected to its source (or maybe a resistor between them) and the load connected drain to source. It acts as a current regulator, wasting all the extra voltage once its current limit is reached. In this case, it limits how much current can pass through the LED in the opto coupler.
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