Hi olio - that is the "seam edge" of that surface.

Basically every NURBS surface is made up of a rectangular grid of control points.

You can kind of think of it similar to a sheet of paper, but it is kind of like rubber paper since it can be pulled and stretched.

A piece of paper has 4 edges on it, and if you want to make a cylindrical tube out of your piece of paper you will roll it so that 2 opposite edges are touching. Because the opposite edges are touching along the same spot it will look like 1 edge, and that is what produces the kind of edge that you show there.

- Michael