Its called fog of war. Many game companys use it. Basicaly aces high renders a set distance from your view point. To see this effect in motion without fog of war, just press Shift F1, F2, F3 to reduce the distance you can see. This in aces high 1, dont know about hitech aces high 2 though.
P.S. Hitech, could you please post a image inside the factorie!?