The Algebraic Subset From Visible () action (available in the Subsets menu of the Plane and Cube plot windows) creates subsets containing all of the points that are visible on the current plot. The subset content of this is the same as for the Subset From Visible action, but this action defines the subset as an algebraic expression based on the axis limits rather than just marking the points that can currently be seen. This can be more useful, for instance if you want to use the subset definition in some other context. However, it is only available for some plot types (Plane and Cube), since in other cases the rectangular visibility region does not correspond to a straightforward algebraic expression.
When you hit this button, the Multi Algebraic Subset Window will be displayed, showing the algebraic function(s) corresponding to the currently visible region, and offering to create a new subset (or, if there are multiple datasets plotted, several new subsets) from it.