Hi Pilou, here is some additional explanation.

Here is the setup for a normal 2-rail sweep:



The 2 rails are flat on the plane (one of the 2 rails is shown selected here), and the profile is sticking up vertically.

You can create a sweep from these curves, by selecting the profile and running Construct / Sweep, then selecting the rails. That will generate this surface (seen from the side here):



There is nothing terrible about this surface, but if you don't like the shape of the sweep as viewed from the side, it is difficult to change this in the current beta. In the next beta, the scaling curve option will give you an additional tool to control the shape as viewed from the side. This shows the side-profile scaling curve (shown here as the selected curve):



In the next beta, this curve can be applied to the sweep by clicking a new "Pick scaling curve" option that shows up along with the other sweep parameters. In this case applying the scaling curve (which should generally run down the center line of the sweep) will update the sweep to have this result, viewed from the side again:



The scaling curve is shown selected there. You can see that this has changed shape from the original sweep shown above - the sweep profiles (vertical sections) have been stretched to match up with the scaling curve. So now the side-view of the sweep has been adapted to have the same type of shape as the scaling curve.

Previously making this kind of an edit would have been difficult, you would have to try to put in a lot of vertical profiles to try and gain more control over the shape in the side view. But even as you added more profiles you would tend to get bumps. This new method does not not cause bumps.

So this helps you gain additional control over the shape of the sweep - it mostly adds control over the shape as viewed from the side-view.

Does that make more sense?

You might think of it as a kind of optional 3rd rail that can run down the center of a 2-rail sweep to provide additional control.

- Michael