This option enables to select a sub-area of interest. An area is selected by left mouse clicking and dragging to establish a rectangular window. With the right mouse button, choose "Selection->Show Selection" to display the area of interest. You might use the zoom option to increase the viewed resolution. The full image can be retrieved by choosing "Selection->Full Image" (see Figure). The sub-area selections can be disabled by un-checking the box "Selection->Allow Selection".
Figure : Sub-area selection menu.