The Hide One Level Up and Hide One Level Down commands are used to hide and unhide levels while viewing a 3D view. For example, if you are viewing all levels of a circuit and select Hide One Level Down, then metal level 0 is removed from the display and level 1 becomes the top level displayed. If you subsequently choose Hide One Level Down, then level 2 becomes the top level displayed, etc. If you choose to Hide One Level Up, level 1 becomes the top level displayed.
These commands are useful when your circuit contains metal that is obstructing your view and you want to remove it to see the metal under it.