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 I_CEO(I_DARK). 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, V_F=0.7V) is applied to the LED at the input side, though it is the OFF area, is defined as OFF-state Collector Current I_{C (off)}. This means that the OFF-state voltage at the input side is at a level below 0.7V.