The physical description file, or PDF, is a template associated with the controller or string that describes its physical characteristics. See Physical description files (PDF) for more details.
In the Create Controller dialog, if the control unit type selected has only one possible physical description, then HCM simply creates the controller when you press OK. If the control unit type selected has more than one possible physical description, pressing OK causes the Choose Physical Description dialog to automatically appear. (You can deliberately open this dialog; see Editing a controller). Select a physical description and press OK to create the new controller.
Changing the physical description affects all controllers within a subsystem (for example, dual frame); you may only change to a physical description for a subsystem with the same or fewer number of components.