The CLIP tool texture creates a clip plane (invisible wall) on all three collision hulls (HULL1-3) that players cannot move through, but can shoot through.

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 players from getting into areas they 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 User posted image

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.