WIDGET_API void       XPSetWidgetDescriptor(
                         XPWidgetID           inWidget,    
                         const char *         inDescriptor);    

Every widget has a descriptor, which is a text string. What the text string is used for varies from widget to widget; for example, a push button’s text is its descriptor, a caption shows its descriptor, and a text field’s descriptor is the text being edited. In other words, the usage for the text varies from widget to widget, but this API provides a universal and convenient way to get at it. While not all UI widgets need their descriptor, many do.