This is what SOF2 lacked. This eliminates most of the Tris or the engine rendering the whole level at one time. This splits. If you are in one room the engine will not render the other room until you are in it. Pretty cool stuff. I saw this same thing in the Red Faction Editor.
www.redrummdk.com/images/zones.jpg

it's like define the properties for nodes. That's awesome.

I know that serious same had zoning too. I didn't notice the effect that had on tris.

Whats cool is that you can see the zones in different colors. I might be worth looking like subtracting a large similated outdoor area and add a bunch of bsp's walls, boxes etc. and see if it can be zoned off in quarters or half. From what I have been seeing the zone portals can overlap other brushes and not cause any problems. I feel another tutorial coming on.