Models will be culled (hidden) when the BBox 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.
$bbox (min x) (min y) (min z) (max x) (max y) (max z)