Troy, the back profile of a bow determines where the limb should bend for best performance for your draw length/bow length. For example, a bow with a rectangular back profile should show a more elliptical tiller than say a pyramid back profiled bow where it's tiller will show more round, or arc of circle.
That being said, an elliptical tiller will stack quicker so better suited for a shorter draw. But the arc of circle tiller is better suited for longer draws. Or at least, that's the principle I use. ART