Hi Andrei - for G2 and G3 that's all about matching curvature properties so that 2 surfaces have equal curvature (for G2) or rate of change of curvature (for G3) where 2 surfaces meet.

This just does not come into play so much when things are only flat planes or straight lines, like in your cylinder case there the direction you are matching is totally flat. That's basically a kind of special case, it's much harder to manually match curvature values when the thing you're matching is bending around instead of totally straight, I think you realized that in one of your previous posts where you were talking about doing a blend between spheres. If you measure your Loft result in that sphere case you're going to find that it's not so easy to make even a G1 result, let alone G2 or G3 just by doing a loose loft.

Even that sphere case is still kind of limited in that it has constant curvature in all spots, if you were to try a fully general freeform surface like cut a hole in your helicopter body and then try to do a G3 surface coming off of that just by using Loft you're going to find that it's just not going to work... It's too hard to manually figure out where to place each section curve in a general case like that, for a cylinder case it's a very limited case and you can just move profiles in one direction for that one. That will only help you if you are only modeling cylinders though, it's not a fully general technique for use with any kind of freeform surface.

- Michael