Extensions are a means to extend maxwell capabilities in several areas: geometry loaders, geometry modifiers, procedural geometry, scene modifiers and procedural textures.
Geometry loaders allow geometry to be loaded at the beginning of the render process. Proxy objects and external referencing can be implemented using these extensions.
Geometry modifiers allow an object to be modified at the beginning of the render process. For example, a coarse geometry object could be refined just before starting the render, depending on some scene setup.
Procedural geometry extensions enable maxwell to render mathematically defined primitives. It is geometry created on-the-fly.
Scene modifiers are able to change elements in the scene just before the render starts.
Procedural textures generate texture colors on-the-fly while rendering.