Controllers and strings come in various physical arrangements. In some cases there are alternate physical arrangements available for the same unit type. HCM uses physical description files (PDF) to represent the non-default physical arrangements of controllers and strings. A default controller is one that has one or more channel interfaces, one segment, and one device interface. It cannot be grouped with other controllers. A default string is one that has one interface and eight devices per unit.
A PDF is an ASCII text file containing one or more (usually one) physical descriptions for controllers or strings. The physical description is a template of an object hierarchy that can be used to characterize a controller (or set of controllers) or a string.