Translation:
-Brushes that make up the outside walls of your map should always be detail0.

-Brushes inside the outside walls should be detail 1.

***Dedit Law*** The two shall NEVER intersect however they may touch.

Detail 0 brushes represent the outside of the world.(walls, roof, floor) Detail 1 represent, "Brushes inside the world" (boxes, terrain, interactive objects, ect.)

What it looks like ingame:

This is an example of how detail 0/1 should be put to use. Frame one shows an ingame render. Frame two shows that the (detail 0) hall is basicly a hollow rectangle with (detail 1) things in it. Frame three shows how those brushes are detailed.

But why?!?!

Because thats how it works on the lith engine. The Detail 0 shell determines what is drawn at any given time. Its is recommended that per scene (What you can see at any given time(in the players peripheral field of view)) be kept around 700 polygons/scene on multi-player levels and 800 or so for single-player levels. Of course a modern computer can handle many more polygons/scene it's a good idea to keep your first few maps around these numbers.

You can have as many polygons as you like in a map if you follow the rules of the HullMaker. When I say 700 polygons/scene I do mean Per-scene. So why not make many scenes?? If you have several areas(scenes) in a map connected with a bent hallway (90 degree bend) the detail 0 brushes you made will tell the game engine the player cannot see around the bend. Thus not drawing any of the map beyond the bend. This is all done automatically. See below:

As shown above, the player is in Room B and has a limited field of view. No matter where he may go in Room B there is no possible way he can see into Room A even looking down the hall because of the bend. Now sticking a bunch of 90 degree bends all over may not look right or fit the, “flow” of your map. The tricky part is using this technique while being creative. The player should not even realize he is wondering around bends. DM_Outpost4 is a perfect example as to how bends can divide a map in to a scenes. In this case 3-4 which can be viewed clearly from overhead.