For IC couplers, some products are defined with a threshold input current, and others are defined with a current transfer ratio. What is the difference between them?

Many IC couplers consist of a photodiode and a photodetector IC incorporating an amplifier circuit and waveform shaping circuit. When input LED current I_F reaches or exceeds a certain value, these products operate logically with the output being inverted. The threshold value for the input current required for inverting the output is called the "threshold input current" and denoted by such symbols as I_FHL and I_FLH. However, some products consist of a photodiode and a photodetector IC in a very simple configuration with transistors only. In this case, the product operates in such a way that the output current changes as input LED current I_F changes, and has no threshold. Like general-purpose transistor couplers, this type of product is defined with the current transfer ratio (CTR).