What is the difference between QwtPlotLegendItem and QwtLegendItem ???

In the documents, I found QwtLegendItem but not QwtPlotLegendItem.
In my Qwt library, I found QwtPlotLegendItem but not QwtLegendItem.
I have
#include <qwt_plot_legenditem.h>
but not
#include <qwt_legenditem.h>


I'm trying to use
qwtLegenditem.setItemmode(..)

but i can not find it. I am using qwtPlotLegendItem

Thank you.