Curve used to create a transform curve
To create a transform curve:
To move the transform curve, click and drag the curve you want to duplicate. To rotate or scale the transform curve, click the parent curve, go to the Curve sub-object level in the modifier stack, and then rotate or scale the transform curve.
When you use Move to create the transform curve, it simply copies the parent. (It doesn't exaggerate curvature as an offset curve does.)
Axis constraints don't apply to the creation of transform curves. You can click to create the curve in place; then once it is created, transform it using constraints.
At creation time, transform curves have no parameters.
At modification time, you can transform the transform curve as a curve sub-object, and you can animate curve sub-object transforms. Transform curves also have one control in the Modify panel.
Lets you replace the parent curve. Click the button, then click the curve to replace the original curve.