Part I: to understand the goal of the software design of Geant4, it is useful to begin by reading the User Requirements Document referred to in the next section.
Part II: ``Design and Function of the Geant4 Categories'' provides detailed information about the design of each class category and the classes in it. Before considering an extension of one of the toolkit categories, a detailed understanding of that category is required.
Part III: ``Extending Toolkit Functionality'' explains in some detail how to extend the functionality of Geant4. Most of the class categories are covered and some, which are especially useful to most users, are covered in greater detail.
It is not necessary to understand the entire manual before adding a new functionality. To add a new physics process, for example, only the following items must be read and understood:
the design principle described in the ``Physics processes'' chapter of Part II
techniques explained in the ``Physics processes'' chapter of Part III.