Each of the two model files can be added during the model checking by specifying an additional file or by reading in directly in SPRING with File 
Import Submesh…
In these additional files can be any number of data which are treated as if they are in the model files. If an attribute exists twice (e.g. a node with two withdrawal amounts) the last value overrides the previous one.
With the additional files you can save disk space in several model types. All non-changing information (such as mesh data, permeability, etc.) is stored in the model file. For different simulations the withdrawal amounts or the boundary conditions can be stored in auxiliary files.
The additional files have exactly the same structure as the corresponding "master file" (*.net or *. 3d).
Attention! When opening a model SPRING reads only the main input files, but never the additional ones. They have to be read explicitly, as described above, or are specified in the dialog of the model checking.