Toggling View > Stitches Sewn Only changes how you view the design on your computer screen as it is sewn.
When View > Stitches Sewn Only is toggled on, you can view the design on your screen as it is being sewn (that is, you can see the stitches on your screen as they are sewn).
When View > Stitches Sewn Only is toggled off, you will not be able to watch the design being sewn. You will see all of the stitches of the design.
You can also access this function by right-clicking in the AMAYA OS window.