Prepare Ligands: stereocenters

Hi all, 

I have a list of ligand to be prepared generating tautomers, isomers and such.

All the ligands have 3 stereocenters, but I want to keep fixed 2 and let the remaining one to be R or S.

 

It is not completely clear to me the options in the tool "Prepare Ligands"

 

Thanks in advance.

 

Andrea