The documentation says "The intersection with the curve segment stores the parametric hit location
along the curve segment as u-coordinate (range 0 to +1)."
As linear curves are represented as cylindrical segments, then a ray will intersect the cylinder in two places. Is the point represented by "u" the closest point of the "ray" to the line? Or is the projection of the 'first' intersection of the ray with the cylinder onto the line?