Under IFRS 9, there are three requirements: an economic relationship between the hedging instrument and the hedged item, credit risk that does not dominate the value changes arising from that relationship, and a hedge ratio that equals the ratio resulting from the quantities actually hedged. ASC 815 requires the hedging relationship to be expected to be "highly effective" in offsetting the changes, both at inception and afterward.
Consider a hypothetical example of a fair value hedge in which the value of the hedged item falls by 100 and the value of the hedging instrument rises by 95. Here 95 is the part that offsets and 5 is the part that does not. In a fair value hedge, IFRS 9 recognizes the gains and losses on the hedging instrument and on the hedged item each in profit or loss, so a net loss of 5 appears there. Since the 2017 amendments, ASC 815 no longer measures and recognizes the non-offsetting part separately.
IFRS 9 does not set a numerical range for the effectiveness requirements. The assessment is made not only at inception of the hedging relationship but also on an ongoing basis.