Fit spline is simply what the name implies - it "fits" a spline over existing geometry thereby becoming a child or slave of whatever it is fitted to (I believe the parametric option is on by default). If you want to modify the fit spline, you can - you modify the underlying geometry. There are times when I have two or three splines that I want to make a single spline. If I know that I'll want to adjust the Fit spline, I dimension the vectors and magnitudes of my underlying splines (or arcs lines etc.) BEFORE Fit spline over them. If you decide after the face (Fit spline) you can use select-other to get to the base geometry to modify or dimension it.