This window is for generating a spectrum that is equivalent to an interpolated line that you have drawn on the plot. Such lines can then be subtracted to or from, or divided into the current spectrum, thus providing a way to background correct a spectrum using a hand-drawn line.
To create an interpolated line select the
Drawing mode->
option in the Graphics
menu and draw out the line. Different types of interpolated lines can be
selected first in the Graphics->Curve type menu. The default is
hermite a type of spline that attempts to connect points in a
similar way to a human.