Master Model file is a term used to describe a parent DGN model file having several child reference DGN model files attached. Within the organization of model files, quite often the 3D master model file (DGN model file attachments) are necessary for the development of all the various design discipline project areas. Attached DGN models typically include an architectural model (more than one usually), a structural model, a site (civil) model, a piping model, a mechanical (HVAC and plumbing) model, and models related to other discipline specific areas.