The tolerance that can optionally be passed to Migrad defines the convergence criterion for the minimization by comparing it to the current estimated distance to minimum (edm). The minimization is stopped when edm < 0.0002*tolerance (see VariableMetricBuilder.cxx of Minuit).
The default value (in Migrad as well as in Pawian) is tolerance = 0.1. A different value can be set by using the minimumTolerance parser option.
-- JulianPychy - 03 Jul 2014