如何理解自动布局约束的第一项和第二项?

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 & second Item .If I use a tableView as a View's subView in xib