It depends on which tolerance type you choose. L1 compares the tolerance to an average of all "changes" across the domain, L2 compares the tolerance to the deviation around the average change, and max compares the tolerance to the max change. "Change" depends on what's being solved (head, solute mass, heat energy). L1 is the least stringent and "max" is the most stringent. Which you choose depends on the complexity of the model: more complex often require more stringent comparison. More complex models also may require smaller tolerance value. To help you decide, run tests on your model. There is no better way to select the right combination of settings.
Pete