What can a butt lift do, and what can't it do?

 I guess it depends on what the meaning of lift is.

If by a lift you mean an elevation of the buttock tissues, that can be achieved by resecting skin and soft tissues from the upper part of the buttocks. 

If by a lift you mean an increase in volume or projection of the buttocks, that can be achieved by a fat injection, or insertion of an implant.

If by a lift you mean a tightening of the skin in the butt, that can be achieved by both techniques mentioned above. The difference is that the first option decreases buttocks volume (because you are resecting skin and fat), and the second option increases the volume of the buttock.

You can read a little more about buttock lift and lower body lift in the link below.