Custom renderers are registered in Softimage via the Init callback, which describes what operations the renderer can perform, which file formats it supports, any properties it provides, and, in the case of exporting archives, which file extension to use.
The renderer plug-in must provide at least one process type, one file format and register a property to use to control the renderer (with the siRenderPropertyOptions enumeration value). If any of these are missing, registration fails.