Many IIXC_Page object methods have a pRect parameter that specifies a portion of the page to be processed. When this parameter is NULL, the 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.