Hi Max,
That parameter doesn't do anything if you have stereo samples. In that case, the stereo field is set by the samples themselves. If you have monaural samples and specify them to play in a stereo rank, Hauptwerk spreads the notes across an artificial stereo field. That is, some notes to the left and some notes to the right. If the distance is small, the perceived angle will seem large. If the distance is large, all notes will seem closer together. Set the value to 999 and the perceived angle is about zero and the rank will seem to be monaural.