There are two programs installed by the instructions in Download and Install. The underlying software library is called Fokko in honor of its primary architect Fokko du Cloux. Fokko worked on the software in 2004 and 2005. Since then Marc van Leeuwen is the primary software developer.
The program atlas is designed for interactive exploration of real reductive Lie groups and their representations. It has been developed by Marc van Leeuwen since 2006. It is built on a programming language axis, also developed by Marc van Leeuwen. The atlas software invokes (invisibly to the user!) the libraries of Fokko to create mathematical objects; then the axis language offers standard programming tools (loops, variables, scripts, and so on) with which the user can manipulate them.
An important part of atlas is the (constantly growing) library of predefined scripts, which can be used or modified to do interesting mathematical tasks. These files are found in the atlas-scripts directory, and have the suffix .at (or occasionally .ax). This library has been developed by Marc van Leeuwen, Jeffrey Adams, Annegret Paul, David Vogan and others.