Func detail
Entity Description
A brush entity that does not retain any actual entity data after a compile, making it similar to a world brush. It is solid (unless materials specify otherwise) and blocks light just like a normal world brush. However, unlike a world brush, a func_detail does not block visibility. Therefore it does not contribute to visibility calculations and does not divide the BSP tree. Thus, they are ideal for optimizing "visual detail" brushes that do not significantly block the player's line-of-sight (like debris, fences, or even small buildings) and complicated brushwork (like stairs, columns and angled brushes).
For an example of how func_detail geometry can be used to reduce face splits, see the example file Steam\SteamApps\username\sourcesdk_content\hl2\mapsrc\sdk_func_detail.vmf.