The default position is a good starting point (.e. 25, 50 and 75). first adjust the four luminosity zones to taste. Having done so, you will probably notice that the amount of adjustment you have applied to each zone differs between them. You then use those three zone demarcation sliders to roughly determine how much of the luminosity scale should be more influenced by the adjustment you made to a particular zone. Now let us assume for example that you have made the lights lighter and the darks darker, but you find the ligher part of the dark zone to be a bit too dark. In that case, you spread the 50 control leftward, because that expands the adjustment you made to the lights into the region of the darks. You will notice the darsks start to lighten as you move it further and further to the left. So in general, what you are doing with those markers is altering the size of the luminosity zone over which a particular zonal adjustment has been applied. There are no rules or guidelines - just "season to taste".