Items are objects in the game which can only be picked up, dropped or consumed, either by a character that can eat it, or as part of a craft. Most items can be stacked up to 10, and each stack takes up one tile.
Packaged Buildings[]
Buildings in item form count as a full stack each. Before a building is set up, it comes in package form; it is then layed out and assembled where it is needed. Buildings can be disassembled at any time by an engineer, and can either be reassembled elsewhere or put back in storage.
Storage[]
Storage constructs organise the minds of your dwarves, to make room for 3 stacks per tile instead of 1.