A sub-set index list contains only a sub-set of the indices of another index list. One example of a sub-set is the built-in radionuclides index list.
One of the main uses of the sub-set index list is to remove unnecessary indices from the computations.
Consider a model which deals with different types of pesticides. Thus, a material is created for each pesticide. Important when modeling pesticides is also the water content, so water transport must be modelled as well.
The equations for water transport are naturally very different from those of pesticide evolution, and for simplicity the model is divided into two parts.
Here two sub-sets of the Materials index list can be made: one sub-set that contains only water, and one containing the pesticides. The equations for water transport are given the Water sub-set index list and the others are given the Pesticides sub-set.