The current with the threshold value for the photo triac to turn ON when the current passed to the LED is increased is defined as trigger LED current (IFT). It varies with the product. The maximum value for such a variation is specified with IFT(Max). However, this does not mean that in circuit designing, LED current IF must not exceed this Max value. In designing IF, , also by considering LED deterioration over time, temperature, and design margin, a value for IF larger than IFT (Max) must be set.
For details, also refer to Toshiba Application Note "Triac couplers-basic properties and application design", section 9.1 How to design the input-side LED current IF. For IF, the maximum rating including temperature derating is also specified, and whether or not such a rating is exceeded must be checked.