The Defense Grid is a virtual area which describes the area of the protection status about Structures and can be visualized by selecting structure that creates them.
Green defense grids are generated around the Matter Converter and the Outpost which enables defensive bonuses on structures inside it and gives them a 30% plus in both kinectic-based and energy-based defense. The grid generated around the Outpost is considerably smaller than the Matter Converter. You can expand a defense grid by building other structures that generates them, like the Defense Relays and connecting them already existent grid.
Yellow defense grids are generated by Defense Relays disconnected from a green grid. It is just representative and will not give any protection to the structures inside it.
Any structure existing outside a defense ring, or a yellow one, does not have any resistance against kinetic or energy attacks. When these vulnerable structures outside a def-ring are attacked, all damage dealt will be deducted in it's entirety from the structure's hit point health, without any of the damage being deflected.