MOCASSIN 2.02 manual
This is the convergence limit for the variation of a given parameter, in each grid cell from one Monte Carlo iteration to the next (e.g. 0.05 means changes of 5% maximum). In the case of a pure-gas (or gas+dust) simulation the criterion is based on the rate of change of neutral hydrogen. In the case of dust-only the criterion is based on the rate of change of the dust temperatures.
Other convergence criteria can also be used, at the moment, this would require a simple editing of some source modules. If you would like to use a different convergence criterion please email me and I can do the editing for you.