From Quake Wiki
They don’t seal the map. To be compatible with existing Quake 1 mapping editors, detail brushes can be added by creating an entity with classname "func_detail". When qbsp reads the map file, it will add any brushes included in a func_detail entity into the worldspawn as details and remove the func_detail entity. Any number of func_detail entities can be used (useful for grouping) and all included brushes will be added to the worldspawn.
This classname used on brushes textured with fence textures will alpha-mask the #255 colored parts of that texture rendering those parts invisible.
func detail variants
func_detail_illusionary : A func_detail variant with no collision (players / monsters / gunfire), doesn’t split world faces and doesn’t cast shadows unless enabled with "_shadow" "1". Useful for creating hanging vines or spiderwebs.
func_detail_wall : This func_detail variant doesn’t split world faces. Useful for when you want a decoration touching a floor or wall to not split the floor/wall faces (you’ll get some overdraw instead.) If it completely covers up a world face, that face will get clipped away, so it’s not suitable for fence textures; see func_detail_fence instead.
func_detail_fence : Similar to func_detail_wall, but its better in that it doesn't use up a static entity and renders as part of the world which is faster. It is suitable for fence textures, never clips away world faces. Useful for fences, grates, etc., that are solid and block gunfire.