The AFP Renderer is an implementation of an output target for Apache FOP that renders formatting objects (XSL-FO) to AFP or (MO:DCA). Documents generated in the MO:DCA format are primarily used for large scale commercial printing.
The AFP Renderer was originally released as an add-on output renderer for Apache FOP. It has since been incubated into the core Apache FOP codebase. There is no need to download the AFP Renderer separately, all you need is the core AFP Renderer.
See https://xmlgraphics.apache.org/fop/1.1/output.html#afp for further details.