The storage hopper can be set to one of four input/output statuses. These are: Add/Remove, Remove Only, Add Only and Locked. The status can be cycled by pressing the 'Interact' key (default 'E'). These status codes only apply to standalone machines, such as a smelter or assembly plant. They do not apply to conveyors or conveyor based assembly machines with the single exception that a locked hopper will not output onto a conveyor.