Specifies how the underlying price is determined at the point of option exercise. The underlying price may be set to the current settlement price, set to a special reference, set to the optimal value of the underlying during the defined period ("Look-back") or set to the average value of the underlying during the defined period ("Asian option").
1 = Regular
2 = Special reference
3 = Optimal value (Lookback)
4 = Average value (Asian option)