The CLIP tool texture creates a clip plane (invisible wall) on all four cliphulls that players cannot move through, but can shoot through. Clip brushes can not exist outside the world, so keep them inside of your map.
func_illusionary based railings with CLIP brushes
They can be used to smooth out walls and terrain so the player doesn't get stuck, or to prevent the player from getting into areas he shouldn't.
Terrain with CLIP brushes
In Counter-Strike, weapons can still be dropped through clip planes. Solutions for weapon and bomb stuck spots include creating a func_detail textured with NULL. Since details are converted to world geometry, models can't be rendered inside of these brushes. You could either move the origin of the model outside of the brush, or convert it into a func_wall.