An external module is a program that interacts with Geomview. A module communicates with Geomview through GCL and can control any apsect of Geomview that you can control through Geomview's user interface.
In many cases an external module is a specialized program that implements some mathematical algorithm that creates a geometric object that changes shape as the algorithm progresses. The module informs Geomview of the new object shape at each step, so the object appears to evolve with time in the Geomview window. In this way Geomview serves as a display engine for the module.
An external module may be interactive. It can respond to mouse and keyboard events that take place in a Geomview window, thus extending the capability of Geomview itself.