Consider charges and check for electroneutrality. If the sum of all charges is not zero, search for mistakes and if there are none, throw in holes or electrons, as your system provides. We take the Schottky defect as a fitting elementary example and go through the movements 1. Charge neutrality demands S pos. But it is the same.
Lets see why.
Now lets look at the energies in the exponent. As always, the energy scale is relative. From whatever zero point you measure your energy to make a Ag vacancy or a Cl vacancy, you must subtract the energy of the AB molecule as measured in that system. Note that as in treatment given before , the mass action law alone does not specify the vacancy concentration, only their product. OK , we do have a problem, but not with the zero.