func_viscluster
func_viscluster is a internal brush entity available in all Orange Box games. It allows you to manually tell VVIS that the visleafs inside its volume can all see each other and should be treated as one. This is useful in large open areas where visibility calculations would otherwise take a very, very long time to complete. Poor use may be expensive for in-game performance. It should be textured with tools\toolstrigger.
To do: Does it reduce map filesize or just increase in-game performance?
Note: This is an internal entity. When the map is compiled by VBSP it is processed and then removed: it does not exist when the map is running. Note: This entity does not actually change the location or number of leafs in a map. Hammer's portal file says this occurs, but commands such as mat_leafvis show the leafs to still be intact.
Warning: The brush should not cross the surface of a water material or an areaportal. If it really does need to be used over water, bring the brush level with the surface of the water on one side, and then create a second, separate func_viscluster on the other side. Never allow a func_viscluster brush to cross a water plane, or very strange things happen.