Compartmentation separates battleships from battlecruisers

How important are bulkheads, armor, and compartmentalization when distinguishing battleships from battlecruisers?

They are central: battleships are subdivided much more heavily, while battlecruisers accept more open internal volume to save weight and accommodate machinery.

The comparison treats a battleship interior as a honeycomb and a battlecruiser as a more open aerated structure. Fewer internal divisions save steel weight and create larger machinery spaces, but reduce the barriers that contain flooding and damage. The recovered volume can hold machinery or fuel, while armored bulkheads still protect magazines, engines, and sometimes larger sections of the hull. On this basis, Scharnhorst, Gneisenau, and Dunkerque fit the battleship category.