The document discusses reference systems for 3D printing. It describes the components of 3D printing ecosystems, which include machines, software, materials, design, and people. It also outlines different types of reference systems needed for various 3D printing business models, including service bureaus, online 3D printing companies, additive manufacturing, and machine integration. An ideal reference system depends on the kind and size of the business and coordinates the various elements in the 3D printing process.