Note: By default you will be viewing the full desktop of the remote computer when session starts, however you can select a custom region and only view that part of the remote screen. This option becomes useful when trying to reduce the bandwidth usage on slow connections.
Click on "Select View Region" and then select the "Custom Region" option.
Click and drag to select a custom region of the screen.
You will only view the selected region from now on.
Note: Repeat the steps above to select another region or reset the view by clicking on "Select View Region" and then selecting the "Desktop" option.