title: How to understand auto layout constraint's first item & second Item?
body: <p>If I use a tableView as a View's subView in xib. And the tableView is fully covered the View. but in the xib's attributor , i can see the two vertical constrains just added : the 1st constraint use tableView's top as first Item , superView's top as second item  , But in the 2nd constraint, it show that superview's bottom was the first item , and the tableView's bottom was the second item . what is the reason of the xib to decide use who as the first item &amp; second Item .</p>