Spline & spine in Mulitsection
A Spline is a smooth curve that is fit through a series of points. It can be 2D or 3D.
A Spine is a curve (line, circle, ellipse, spline, etc) that represents an finite number of planes that are perpendicular to the curve along it's length.
Splines can be used in Multi-Section surfaces as section curves, guide curves and/or the spine curve.
A spine can be used in a Multi-Section surface to define the direction of the surface as it evolves through the section curves.