When input current IF reaches or exceeds a certain value, an IC coupler with amplifier circuit operates logically with its output being inverted. The threshold value for the input current required for inverting the output is called the "threshold input current" and is denoted by such symbols as IFHL and IFLH. This value varies depending on the product, and the maximum value for its variation is specified, for example, with IFHL(Max). In designing IF for actual use, also by considering LED deterioration over time, temperature, and design margin, a value larger than IFHL(Max) must be set for IF.
Also refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of IC Couplers", section 2.3.1.