The Fit spline function can be found underneath Menu/Tools/Spline Tools/Fit Spline. The way that it works that gives the user the best of both worlds is a method that allows for creating shapes using analytical geometry that can then be dimensioned and parameterized in a logical way. Once the geometry is define this way, A Fit spline function can be attached to the selected geometry thereby “fitting” a C2 or continuous single entity over the analytical geometry. The ultimate benefit of this is that when a surface is created it is smooth and continuous but can still be controlled with simple dimensions.
Using the Fit Spline tool is as simple as selecting the sketch geometry and then assigning a tolerance (or accept the default tolerance). Adjusting the tolerance allows for smooth result or a tighter, more accurate fit but less smooth. When “underlying geometry changes, the Fit Spline follows thereby allowing you to change the spline’s shape using a simple dimensioning scheme.
I encourage you to take a look at how the fit spline can make you life easier.