What are margins in Storyboard, and why does the iPhone 6 plus have different side margins than the other iPhones?

Question

When creating constraints in my views in Xcode 6, I noticed this "constrained to margin" box which is ticked by default:

The result of keeping this box ticked is that I the app would run normally on all iPhones except iPhone 6 Plus where I would have some margins on the left and right of the view:

I tried looking at Apple's documentation, but maybe not hard enough. What is margin in Xcode 6 and above? Why is the margin only on the sides and not on the top/bottom? And why is it that only iPhone 6 Plus has this margin? Why not iPhone 6 and others? Are those margins documented somewhere?

Answer 1

You need to click "relative to margin" in the horizontal space constraint attributes