his function allows you to notify user about changing a volume when packing files.
void __stdcall SetChangeVolProc (HANDLE hArcData, tChangeVolProc pChangeVolProc1);
pChangeVolProc1 contains a pointer to a function that you may want to call when notifying user to change volume (e.g. insterting another diskette). You need to store the value at some place if you want to use it; you can use hArcData that you have returned by OpenArchive to identify that place.