that is pretty simple. the frame data specifies how many times a move with multiple hits prorates.
if a move with multiple hits prorates only once, then the 1+nth hits all suffer from the proration. 10x100 dmg, p2 of 90(1), for example, would do effective damage of 100,90x9 within a combo.
outside of a combo, it would be more like 100, 63x9. (character combo proration value)