From Xojo Documentation
Divides the rect into two rects at the given point on the X axis.
For example, imagine a rect that is 200 pixels wide by 100 pixels tall. With a call to SplitHorizontal(50), the rect will become 50 pixels wide, and a new rect will be returned that is 150x100, positioned exactly to the right 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 width.