I've made a few modifications to better illustrate how the Peering system works (i.e. abstraction from underlying 3D API).
There are a few "gray" areas to Xith3D, i.e. shadows and collision - these are the parts David created and are more or less abandoned. Personally I don't recommend using them unless you understand it (or are willing to learn) enough to maintain it yourself. The rest of the API however is well maintained, i.e. the scenegraph, shaders, ASE loader, etc...
The renderer is fairly complex when you look at it for the first time, mainly due to the 3D API abstraction. I hope my diagram will help shed some light on it.