Caulk

The Caulk texture is ignored by the game even though you can see it in the editor. When applied to a brush face, that brush face is not drawn in the three-dimensional world. Also, because no texture is loaded, Caulk reduces the amount of used memory on the computer when playing, thereby creating a faster, smoother running level in the game. This makes it perfect for places that can't be seen by the player, such as the areas behind walls, between brushes that are butted against each other, and so on.