“Trigger LED current” means “LED current that triggers change in the status”.
IFT, IFLH, IFHL, etc., are used as symbols.
The trigger LED current indicated in the datasheet is the value specified by the product. For stable operation, the designer must design so that at least the trigger LED current (maximum) flows.
The input LED current IF is gradually increased from 0 mA,
If the output shifts to the on state at 1 mA, IFT = 1 mA.
In the following data sheet, the IF required to shift the output to the ON state
It means that the maximum value is 3 mA.
Trigger LED current is an important item for circuit design and lifetime design.