The ME Storage Bus, when attached to an inventory, lets you include in the ME Network other types of storage, ranging form vanilla Minecraft's chests to other mods' storage, such as Factorization Barrels, or Iron Chest's Diamond Chest. The Storage bus also contains an interface which lets you configure which items it's allowed to store. This way, you can configure it just like you can configure storage cells with the ME Preformatter.
Items in chests or barrels linked to the ME Network via the ME Storage Bus can be accessed freely by the entire ME Network and do not require any power (units) to be stored indefinitely. This allows for free (power) storage of items that can still be used by the ME Network.