pv: A pointer to private data that is created and owned by the form receiving the callback.
idOption: An identifier used to disambiguate items in the object that contains the items to be selected. The identifier will vary from form type to form type. For example, a list form would pass the item number of the item that has been selected, while a popup menu may pass the menu ID of the particular menu item that has been selected. More complex forms may choose to pass in data from an attached model to identify the selected item.
typedef void (*PFNSELECT)(void *pv, int idOption);
To register a selection callback routine an application would call IForm_SetSelectHandler(), passing in a pointer to its PFNSELECT function and a pointer to the private data it wishes to access each time the function is called (basically, the same fields that appear in a SelectHandler data structure).