If this option is on, the G1, G2 or G3 CVs along the edges of the Input surface move so that the ratio of the start and end tangent (or G2 Curvature, or G3 Curvature) lengths of the Input surface matches that of the Master. The inside CVs also move to mimic the hull shape of the Master.
When matching surface edges the CVs on both surfaces naturally align. However, when matching to the interior of a surface, or to a trimmed edge, the CV layout may be mismatched, and so a different method is required. The Vector and Project options provide vector-based control over the alignment direction of CVs.