The nucleophiles used are typically generated from precursors (pronucleophiles) in situ after their deprotonation with base. These nucleophiles are then subdivided into "hard" and "soft" nucleophiles using a paradigm for describing nucleophiles that largely rests on the Acid dissociation constant| of their conjugate acids. "Hard" nucleophiles typically have conjugate acids with  greater than 25, while "soft" nucleophiles typically have conjugate acids with  less than 25. 
This descriptor is important because of the impact these nucleophiles have on the stereoselectivity of the product. Stabilized or "soft" nucleophiles invert the stereochemistry of the -allyl complex. This inversion in conjunction with the inversion in stereochemistry associated with the oxidative addition of palladium yields a net retention of stereochemistry. Unstabilized or "hard" nucleophiles, on the other hand, retain the stereochemistry of the -allyl complex, resulting in a net inversion of stereochemistry.
This trend is explained by examining the mechanisms of nucleophilic attack. "Soft" nucleophiles attack the carbon of the allyl group, while "hard" nucleophiles attack the metal center, followed by reductive elimination.