I believe they mentioned in graph that they used union node because the old difference node was deprecated, something like that. anyways i use difference without union and it works well for me in all the cases
Also about Mainspline and Active_spline, do you mean using filter by tag after any get spline data node? if yes, in get spline data we set something like “source” where we are going to look for the spline, with filter by tag we choose exact spline we want, we can avoid using filter by tag, but then it will output data from all the splines from our source