Where to start

You can see some important classes like:

ARCSFactory, a singleton handling all component libraries;
ARCSAbstractComponent, the class describing component behaviours;
ARCSAbstractFamily, the class used to describe exogenous component systems;
ARCSTypeFactoryTemplate, the template used to make it possible to recognize new data types in the engine;
ARCSXMLHandler, the class that parses most XML files used in ARCS;
and manpages for arcsengine, arcslibmaker, arcslibdump, arcpkg, arcsbuilder, arc1to2.

Developping new extensions to ARCS

The ARCS engine supports the following extensions:

Definitions of new types (see arcsinternaltypes.h for an example and ARCSTypeFactoryTemplate for theory);
Definitions of arrays of new types (see arcsarray.h);
Definitions of new components (see classes Loop, Viewer, DisplayInt, StackExploder as examples);
Definitions of qt components with very specific behaviours (see classes ARCSQDynamicObject, ARCSGeneralLogger);
Definitions of family of components to integrate other component systems (see classes ARCSInternalFamily as samples) ;

To see how to embed the ARCS engine into your applications have a look at tools/engine/main.cpp