Many methods IIXC_Page object have pRect parameter that specifies a portion of page to be processed. When this parameter is NULL, entire page is processed. Some languages, like C#, cannot easily pass NULL and they need to pass a valid RECT that covers entire page, and this property may be very helpful there.
HRESULT get_Rect([out, retval] RECT* pRect);
See also IIXC_Page.