Hello Chris,
every 3d surface has a corresponding 2d (uv or parametric) representation. In order to represent the cylindrical surface in two dimensions imagine you cut it through, unroll and put on a plane. The edge that was created as a result of cutting is your line.
To understand more about geometry an topology in OCCT, have a look here:
http://opencascade.blogspot.de/2009/02/topology-and-geometry-in-open-cas...
You can also use Total Engineer to see the topological structure of your shapes:
http://www.tes-cax.de/index-en/download.html
Hope this helps
Commenter-1