The Union framework, originally contributed by Mads Bertelsen for McStas and ported to McXtrace, is a set of components that together allow the description of complex, possibly nested or overlapping, sample environments with multiple scattering, by separating the description of geometry (where matter is) from physical process (what happens to a photon there, e.g. Compton or Rayleigh scattering, photoelectric absorption, or powder diffraction). For a conceptual introduction to the framework – volumes, priority, masks, exit volumes, and the underlying propagation algorithm – see chapter 14.18. This chapter documents the input parameters of every individual Union component.