The container will call -autolayoutDefaultHorizontalBorder of the view to get its (default) horizontal border, and -autolayoutDefaultVerticalBorder to get its (default) vertical border.
You can later change the borders manually by calling the appropriate method of the container, such as -setHorizontalBorder:forView: for boxes.
Renaissance AutoLayout adds a category to NSView, implementing the following methods:
The default implementation of those methods in NSView returns 4 for both of them. Containers and spaces have implementations of those methods returning 0 for both of them.
Unless you are looking for special effects (or for trouble), you
generally don't need to modify borders.