Hi ed - the scaling rail part is a feature of Sweep that you can use for additional control over profile shaping.

It's an additional rail curve that you draw in and then activate by clicking the "Pick scaling rail" button that is one of the sweep options.

Here's an example - here I have 3 profiles that are going to sweep using the line as the rail:



You can see that it will initially have the kind of "ease-in/ease-out" blending from one to the other that you were mentioning:



I can now draw in a scaling rail like this to control the shaping of the sweep further:



Now when doing the sweep, if I click the "Pick scaling rail" button and click that other shaping curve as the scaling rail, it will produce this result:



The scaling rail will also be affected by the "Maintain height" option.

Basically if you use a scaling rail, the profiles will be scaled out from their current spots until they hit that rail - they'll scale in just one direction if "Maintain height" is on, or they'll scale uniformly if "Maintain height" is off.

But there you can see how the scaling rail can be used as an additional control over the shape of the sweep as it travels along the rails, and if the default blending is not working well for you then you can draw in a scaling rail to control it and shape it how you want instead.

- Michael