Minecraft wiki entry for Hopper, associated text: When multiple sorters are tiled directly next to each other, it is usually desirable to have overflow protection. In an overflow safe sorter, even if the topmost hopper has filled up due to an "overflow," the redstone signal strength will not be great enough to interfere with adjacent sorters. In the sorter shown, 1 full stack of items and 4 junk items will produce a signal strength of 3. This is just strong enough to unlock the bottom hopper without affecting adjacent hoppers. If the signal strength were to get up to 4 however, the adjacent hoppers would be unlocked causing the whole system to break down.