With spacing, you set:
the area between the Elements in a Box element (spacing Around).
the area between the outer limit of a Box element and the Elements it contains (spacing Inside).
Spacing is measured in pixels (PX).
For both spacing Around and Inside, you can switch between:
Having a vertical and a horizontal spacing size, or
Having four separate spacing sizes.
You do this by clicking the icons on the right side of the Spacing section.