Extract Ability replaces whatever item the defeated enemy was suppose to drop with Ability Sphere(s). One Ability Sphere is award for using Extract Ability, but two will be awarded for an Overkill. It is not required to Overkill an enemy with Extract Ability, as long as they defeat the enemy in the end with an Overkill, they'll get two Ability Spheres. If the defeated enemy was dropping Ability Sphere(s) for its normal drop, Ability Sphere obtained from Extract Ability will be added to the total. The Distill Ability status doesn't stack; inflicting a different Distill status will remove the current status.