Suppose you want to change this bicycle frame into a step through design. The key is to lengthen the amount by which the seat tube extends above the top tube. That is controlled in this field which is currently set to 35mm. As we click the extension icon, we can see that this extension can be controlled by a distance along the edge of the seat tube, or by a distance along the centerline of the seat tube. Either option will work. Regardless of how we define the extension, it will need to be a lot longer than what we currently have.
Before we change this value, we should check and see how we are controlling our seat tube length. As you can see, we are currently controlling the seat tube length by center to center measurement. In other words the distance from the center of the bottom bracket to the center of the seat tube - top tube intersection. This means that if we were to lengthen the extension above the top tube. The top tube would remain fixed in it's current position based on seat tube (c-c) length, and our seat tube would just get a lot longer. Because we would like the total length of seat tube to remain at the current length, we should define the seat tube by center to top, in other words the distance from the center of the bottom bracket to the top of the seat tube. If we keep this value fixed by choosing the seat tube (c-t) option, when we increase the extension of the seat tube, the seat tube will remain the same length, but the top tube will shift down along the seat tube.
Another thing to be aware of when setting up a step through frame is how the front end is defined. Because our front end is currently defined by front center distance and head tube length, we can safely shift the top tube up and down along the seat tube without altering the front end geometry. If our front end were controlled by handlebar position in X and Y coordinates from the bottom bracket, or by stack and reach, we would see that our front end remains similarly fixed as we shift the back end of the top tube up and down along the seat tube. If on the other hand we define our front end by effective top tube length and top tube angle, we will see that as we shift the back end of the top tube up and down along the seat tube, our head tube will grow longer and shorter. The reason for this is that we've defined a specific top tube angle. BikeCAD must maintain this angle no matter how other dimensions on the bike are changed.