User-defined contact models are written in the C++ language, and compiled as a DLL file (dynamic link library) that can be loaded whenever it is needed.

The C++ language makes it easy to add a new contact model or a new constitutive model compiled as a DLL (Dynamic Link Library). Using the “virtual function” support provided by C++, it is possible to have a user-written function install itself into a model library at runtime. The file containing the function is completely self-contained—the model name, property names and local data structures are all encapsulated in the file. However, once the model is installed, the host program can refer to the specific model names in commands such as MODEL, PROPERTY and PRINT. Further, the model and its associated properties can be manipulated by FISH in a way similar to the way in which the built-in properties are available to FISH functions.

Detailed information on the implementation of contact models (zip version)
Detailed information on the implementation of continuum models (zip version)