In forge there is something called a "soft safe zone" this customizable box determines your map boundaries, ie. anything OUT of the box is treated as a hard kill zone, end of statement. So it appears that bungie gave us the soft safe zone to section off our maps in general, and gave us the other kill volumes to form "pockets" of death inside the general area of the map where we need them. If this was otherwise posted already I apologize, If not, hope this helps.