Burning out cleanly means that it can be used like wax in lost wax casting. The part you model in Castilene can be enclosed in a ceramic shell or plaster-based mold, fired in a kiln until the model is gone, and then the resulting void is filled with molten metal. You can't do that with regular modeling clay (like NSP), because the clay in it won't combust; it just sits there. But you can pull a rubber mold from it, cast wax in that, and burn the wax out.
You can contact Chavant by email through their site here: http://chavant.com/castilene/index.html