They are the same, they just have different names.
It's useful when you are making composites and want to have an empty fill (like studs) in a material that is next to an air space. IF they were the same fill, Archicad would remove the line between them.
Also, if a material has Empty Fill assigned in (IE) vectorial hatching it will actually have a fill, whereas Air Space has no fill whatsoever. See attached.