The Subset From Visible () toolbar button creates a new subset containing all of the points that are visible on the current plot. When you click the button, you will be asked for a name for the new subset.
This option can be useful if you have panned, zoomed, set the axes manually, or otherwise navigated the plot so that only the points currently visible are the ones you are interested in. However, it is only suitable if the group you are interested in corresponds to a rectangular region in the plotting space.