From Xojo Documentation
Divides the rect into two rects at the given point on the Y axis.
For example, imagine a Rect that is 200 pixels wide by 100 pixels tall. A call to SplitVertical (50) on this rect, the rect will become 50 pixels wide, and a new rect will be returned that is 150x100, positioned exactly to the top of the original rect.
This example is taken from the Paint event of a Canvas. It takes a rect and splits it horizontally at the midpoint of the height.