Unreal Tournament III Tips and Information
This information is provided by DGUnreal.  Please do not copy this and post it elsewhere.

Additive mode versus Subtractive mode

The Unreal Engine 3 is always in Additive mode.  There is no real Subtractive mode like UE2.
Say that with me again… there is no subtractive mode…  there is no subtractive mode.

Choosing Subtractive mode when creating a new map simply inserts a massive 512k x 512k x 512k world-size blue CSG Add cube into the always-additive map. This cube is not fully optimized, so you will see longer build times if you decide to choose Subtractive mode.

To optimize the world cube in a Subtractive mode map, simply vertex-edit the massive 512k world-size blue CSG cube down to slightly larger than the size of your map play area. You can then select the surfaces of this brush and set their lightmap properties and Material to a more optimal setting.  An optimal Lightmap Resolution value would be 65536, optionally with Accept Lights and Accept Dynamic Lights both unchecked, and an optimal Material would be EngineMaterials.BlackUnlitMaterial or something similar.

So how do I create an “Indoor” map then?

To create an indoor or mixed indoor/outdoor map, simply add an Additive cube as the first brush in the editor that is just slightly larger than the play area. If you are unsure of how large the final map will be, simply add a large cube such as 8192x8192x8192 or 16384x16384x16384 to start off with. This initial add cube brush can then be vertex-edited to the proper size when the map is close to completion.
If the indoor style map includes holes in the ceiling, a Skydome can be used to provide the sky.

Change Subtractive to Additive

To “change” the current map from Subtractive mode to Additive mode, simply delete the blue Add cube that is at the outside edges of the world grid.

Change Additive to Subtractive

To “change” the current map from Additive mode to Subtractive mode, simply add a massive 524288 x 524288 x 524288 blue Add cube, which is the complete world size, then right-click on the cube brush and choose Order -> To First.

With permission, I am hosting this information for the UT 3 editor.  A big thanks to DGUnreal for allowing this useful information to live on.  I duplicated the tutorials as they were given to me.  There may be information that was going to be added by DGUnreal. But take this information on this page as completed.