title: Define position for SCNNode
body: <p>I create the <code>SCNBox</code> based on the <code>SCNVector3</code>. But I don't understand the concept of the creation. </p>
  
  <p>For 2D rectangle, the <em>x</em> and <em>y</em> represent the rectangle top right corner.
  For ARKit 3D <code>SCNBox</code>, what is the <em>x, y</em> and <em>z</em> represent for? Is that represent the center of the <code>SCNBox</code>?</p>