The terminology in this discussion distinguishes a cruiser-hunting 'battle cruiser' from a 'battlecruiser' intended to fight comparable fast capital ships. A fast battleship must fight battleships while having enough speed to prevent battlecruisers escaping. The engineering distinction depends on the power available for a given machinery weight. Once engines provide sufficient power without consuming so much of the displacement, designers can use the released weight for armour and subdivision. That makes a fast battleship feasible without the same sacrifice of protection.
Notable Point · 1:40:31-1:41:13
How propulsion technology separated battlecruisers from fast battleships
Mission and machinery weight explain the transition from cruiser-killing battlecruisers to fast battleships able to fight the battle line.