$bbox
$bbox is a QC command available in all 
Source games. It defines an axis-aligned bounding box used by the AI to detect collisions with the world, mostly for movement purposes.
Models will be culled (hidden) when the bounding box is no longer in the players field of view, as a means to optimize the game. Too small bounding boxes can cause a model to disappear before it really left the FOV.
This is also called the hull.
$bbox lines are automatically generated when compiling a model, as long as there are no lines existing already.
Syntax
$bbox (min x) (min y) (min z) (max x) (max y) (max z)
