The "Help window mode" options let you influence how the HTML Help API is
invoked from the File window. If you choose Owned (the default), the
hwndCaller argument is set to the main window of the application. If you
choose Sibling, hwndCaller is set to NULL (zero). And if you choose Child,
hwndCaller is set to the handle of a separate MDI child window.
Note that these choices do not influence which window type you choose.
In order for the mode to be represented properly you must select an
appropriate corresponding window type. Child mode requires the WS_CHILD
window style, while Owned and Sibling modes require that WS_CHILD not
be set.