In iOS 8 you now have the option to define your constrains relative to a predefined margin to the superview's bounds, instead of the superview's bounds themselves. Yes, it is totally related to the layout margins you pointed to in the docs. One advantage is that you may redefine your margins dynamically, or differently for each kind of device, and the layout will be updated correspondingly without modifying the constraints.
When to use it: when you want to take advantage of this new flexibility.
When to NOT use it: for any app targeted to run on iOS 7 or below.