Illustration #1 - Exploring the Options
(When you have your Cutout Window showing, move it away from your canvas so you can see what is happening. Also, do not use OK to set the cutout. If you do, when we move on, use [Control+Z] to remove the experiment and then you will be ready for the next one.)
Start with Offsets both = 0 and Blur = 0 Shadow color = Black and note that these values remove any black Shadow from the Selection.
(1) Now to just see how the shadow works, change the values of the Offsets one at a time. Size does not matter though the larger the size the larger the shadow. Also note by varying the plus/minus values you can get the shadow to move around the shape (sort of).
(2) Set Offsets to about 4 (Still only working with black). This time vary the Opacity to see how it affects the shadow. (The lower the setting the lighter the shadow.)
(3) Return Opacity = 100 and now vary the Blur setting to see how it works. Note that the higher the blur, the more area is shadowed.
(4) Vary both the Blur value and the Opacity value. Notice in combination one can soften the effect creating some neat options.
(5) Now set Offsets both = 0 and use high value for blur and vary opacity to get a neat effect. This specific technique creates a 3D look to the shape. (I use this one often.)
(6) Cancel the Cutout. Then use Selections || Invert to invert the selection. Then, use Cutout again with offset = 0 and blur very high. Vary opacity for another very neat effect. This effect creates a 3D look surrounding the selection.
(7) Now, Play with the Offset values and lower the blur value considerably to get another very neat effect. In one easy step you have create a buttonized frame with a raised looking selection.
Note: With just these few steps, you have numerous ways to either cutout a selection (a shape) or create interesting shadows that give a 3D look to a canvas. You could add textures to the background color or just add texture to inside the shape, or just outside the shape.