It's supposed to be a normalised flight hours count, where the FI accumulated during missions of type X is weighted according to the average fatigue spectrum for that mission. So 1 hour of S&L A-to-B will accumulate less FI than 1 hour of ACM, say.
IIRC, FI 100 was supposed to be "design life" but various fatigue extension programmes, and some early fatigue
scuppered that as anything but a guideline.