It's a setting that allows you to compensate for the fact that as you pan a track from left to right (or R to L), as it passes through the center position, the level will appear to increase, which is undesirable.
Setting the panning law value will drop the level accordingly so as to minimize the difference.
The reason for the different values has to do with mono compatibility -- in mono, the compensation value will be different.
Interestingly, the most effective value -4.5 (which is the mid-point between -3 and -6) is usually the best compromise between the panning law effect in stereo and mono-compatibility, but never seems to be a selectable option!