Storage Crates and Lockers

Crates and Lockers
Crates and Lockers function similarly to chests. Lockers are essentially the same, but to get a double locker they must be stacked vertically instead of horizontally. A storage crate is another storage option, with 18 slots each, sharing their inventory with all other connected crate blocks. This type of storage is not like your neatly organized chests, items put in here will be scrambled around. Each time a player opens them, they will see a randomized selection of items, up to a double chest. You can shift-click more items in even if there's no empty slots in the interface, as long as there's still room. The little bar at the top right shows you how full the crate pile is.

Source : http://www.attackofthebteamwiki.com/wiki/BetterStorage#Crates_and_Lockers