This section of the documentation provides an overview of the structure of ISOFT and an explanation of the design choices made. It is limited to a discussion of code architecture, with numerical methods already having been described in the previous section. A number of design patterns were consciously used when developing ISOFT. The names of these are indicated in the text by small-caps. An understanding of object oriented programming techniques in general and object oriented features in Fortran, in particular, will be useful when reading these notes; see, for example, Rouson, Xia, and Xu (2014). Familiarity with Universal Modelling Language (UML) diagrams will also be helpful (Rouson, Xia, and Xu, 2014). Note that, in the UML diagrams in this section, names of derived types are shown in camel case, as is the convention for class names in most object oriented languages. However, as Fortran is case-insensitive, the decision was made to use underscore separation within the code.