Spline to smooth surface

i want to make a smooth surface with the shape of a spline, as if extruded at right angles to the spline’s curvature. or like a loft between two copies of the spline. how might this be done?

objective is to slice the object below along that curve

Is this the sort of result you want:

If not please explain further.

yes if the cut is a spline and not just a plane.

The Spline was Sketched on a Plane above the Body

Then three Lines were added to the Spline to form a Closed Curve:

Highlight the Closed Curve and drag it downward and through the Body in one movement:

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super. thanks. i feel i should have known that but these tools aren’t second nature yet.