Func detail

When the map is compiled, this entity will intentionally not cut, nor will it create extra unneeded brush faces if the brush intersects with a world brush.

Commonly ed things are: Obviously, use your own judgement too, and experiment to find what works.
 * Stairs
 * Round things, or any other brush with a large amount of sides
 * Any small or thin brushes
 * Brushes in a 3d
 * Generally, whatever doesn't contribute to the main structure of the map