Uplift is the change in a client's chance to convert caused by the campaign: the conversion probability if treated minus the probability if not. Targeting the clients with the highest predicted uplift spends the budget on people the campaign actually moves, not on those who would buy anyway.
Estimates come from the observed treatment vs control conversion rates inside the targeted group, times its size, on - X5 RetailHero clients the model did not train on. Scores are precomputed; this page calls no API.