The physical description file, or PDF, is a template associated with the controller or string that describes its physical characteristics. See Graphical representation of physical objects for more details.
You can change the physical description for a string. Click on the Change Physical Desc… button in the Edit String dialog to open the Choose Physical Description dialog. When you select a different physical description, the existing I/O devices under the string may be reassigned to units. New units are created as needed; there may be empty units left at the end of the string.