All fields and areas in Freewar that have positive Coordinates belong to the Surface. That also applies for Narubia, the Frozen Island and the hovering island Itolos, although they are not connected to the continent. Areas that are below the Surface are called Dungeons. They have negative Coordinates. Places above the Surface (Aviata for example) do also count to the Dungeons. You can see the Coordinates above your Map.
In many Situations it is important to be at the Surface of the World. Weather effects only work at the Surface, the Clan Distress Call and some Spells can only be used at the surface, for example the Excavation Shovel. You can only read the Shout Chat and participate in it if you are on the Surface.
Several Plants grow on the Surface (see Herb).