What is the threshold input current?

When input current I_F 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 I_FHL and I_FLH. This value varies depending on the product, and the maximum value for its variation is specified, for example, with I_FHL(Max). In designing I_F for actual use, also by considering LED deterioration over time, temperature, and design margin, a value larger than I_FHL(Max) must be set for I_F.
Also refer to Toshiba Application Note "Basic Characteristics and Application Circuit Design of IC Couplers", section 2.3.1.
(https://toshiba.semicon-storage.com/ap-en/semiconductor/knowledge/application-note.html)