You can activate this option by going to the Ultimate Affiliate Pro Dashboard -> General Settings -> General Settings tab.
This feature allows you to choose which amount is used if there are multiple ones set for the same action. A good example would be ranks & product rates, your rank could give you 20% but the current offer gives you 50%. If you select the Biggest Amount, then the offer will be in effect since 50% > 20%, but if the Lowest Amount is selected, then it will remain at 20% despite the existing offer.