PSZClassName is either an application-specified name (as defined by the WinRegisterClass call) or the name of a preregistered WC_* class; Preregistered class names are of the form #nnnnn, where nnnnn is up to five digits corresponding to the value of the WC_* class name constant.

This function provides information that is needed to create a subclass of a given class (see WinSubclassWindow).