Detail blocks don't affect the BSP tree, as normal blocks do. They can be used for small details in the level, like small pillars, ornaments, rounded arcs or other elements which would otherwise increase the portal count and building time. Because Detail Blocks do not suppress the rendering of the scene behind them, they should not be used for objects that hide much of the level - otherwise they will reduce the frame rate. Do not try using Detail Blocks as level boundaries, and don't let them touch or intersect other blocks. They are ignored by the collision detection.

Oh, and detail block must never intersect any other block, I forgot where did I read that.

There are some rules of using detail block:
Detail blocks don't affect the BSP tree, as normal blocks do. They can be used for small details in the level, like small pillars, ornaments, rounded arcs or other elements which would otherwise increase the portal count and building time. Because Detail Blocks do not suppress the rendering of the scene behind them, they should not be used for objects that hide much of the level - otherwise they will reduce the frame rate. Do not try using Detail Blocks as level boundaries, and don't let them touch or intersect other blocks. They are ignored by the collision detection.