The Smoothing parameter value in the Construct Filter Feature dialog box (Insert | Feature | Constructed | Filter) determines the degree of smoothing applied to the data for the spline, uniform, and triangle filters.
For the spline filter, it is best to set this value to 0 to indicate that the value computed via GCV should be used. The computed value then replaces the 0 in the Edit window.
For uniform and triangle filters, the smoothing parameter represents one-half of the width of the window used in the (weighted) moving average.