Applicable to Version 2009 and Before
Design files contain default information for the system units, pipe specifications, and fluid zones. If the same units, fluids, and pipe materials are used from one project to the next, rather than defining these properties each time a new project is started, the design file can be opened and these properties automatically imported to ensure continuity between projects.
Design files can be saved at any time during a project by going to the "File" menu ... "Save Design File" ... then giving your design file a name. The design file will be given a ".pdes" file extension.
For example, suppose that you design a lot of HVAC systems that use metric units, water, and steel schedule 40. Once these properties are defined for the first project, save a design file named "HVAC metric.pdes"
When it's time to start a new HVAC project, on the System Settings & Properties dialog box near the bottom, click on the yellow folder under the "Design File" section, then select the "HVAC metric.pdes" design file. The units, specifications, and fluid information are automatically loaded for the new project.
Version 12 and Above
The ability to create design files was removed in version 12 and replaced with the Project File feature.