A scale tool allows data points to be manipulated on the spline. These data points alter the scale of the generated mesh along the spline.

You can access it, by selecting the Spline Mesher component object. After which, this option will appear:

With the tool active, a green circle appears when you hover the cursor over the spline curve (this functionality stems from Unity’s editor, so visual clarity can’t be improved!)

You can clear the scale data by accessing the context menu option