What is the difference between a dark current and OFF-state collector current?

A leakage current flowing between the collector and emitter of the transistor at the output side of a transistor coupler is defined as Dark Current ICEO(IDARK). In this case, a current is not passed to the LED at the input side.
On the other hand, a leakage current flowing between the collector and emitter of the transistor at the output side when a low voltage (In Toshiba's case, VF=0.7V) is applied to the LED at the input side, though it is the OFF area, is defined as OFF-state Collector Current IC (off). This means that the OFF-state voltage at the input side is at a level below 0.7V.

Dark Current ICEO(IDARK) 
Dark Current ICEO(IDARK)
OFF-state Collector Voltage IC(off)
OFF-state Collector Current IC(off)
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