When you write an IMPORT statement in your model, the 'import handler' mechanism is invoked.
This will search your ASCENDLIBRARY path looking for files with recognised extensions that can be loaded to provide additional external library ASCEND functionality, such as blackbox external functions, external methods, external Integrators and external Solvers.
The import handler mechanism allows various types of files to provide these things. The ExtPy mechanism has already been used to allow Python scripts to provide external method functionality.
See also ascend/compiler/importhandler.h