Wolfenstein: Enemy Territory Documentation
© 2003 Splash Damage, Ltd. All Rights Reserved.

Appendix C: Common Textures
A structural and invisible shader. Used to create the structural 'hull' of the BSP. Standard Clip brush which will block players.
No bullets or projectiles are blocked.
Forces additional portals in the BSP. Based on the standard clip brush. Blocks all projectiles.
Allows players to climb up or down. Acts like player clip. It must be present on all sides of the brush. Based on standard clip brush. Blocks all projectiles and produces a metal sound when hit by bullets.
Defines a volume where the lightgrid is active. Any space outside is not calculated. Based on standard clip brush. Blocks all projectiles and produces a wood sound when hit by bullets.
Does not draw in game and is non solid. Used for single sided brushwork like alpha channel textures. Terrain metashader with grass step sound.
Used by brushwork entities for defining their origin point in the map. Terrain metashader with gravel step sound.
Skip is a nonsolid, invisible, structural shader that does not create BSP splits or portals. Used in conjunction with antiportals or hint faces. Terrain metashader with snow step sound.
Used on brushwork trigger entities. It's not drawn and is not solid.    

Please note that this is not a complete list of texture/shaders from the common directory but a summary of the most useful ones for producing a map for Enemy Territory.