What makes a fast battleship rather than a battlecruiser

How much armour would a 1910 ship with ten 12-inch/56 guns need to count as a fast battleship rather than a battlecruiser?

About 11 inches is suggested, provided the protection has broad coverage and the hull has battleship-grade subdivision.

For a proposed 1910 ship with ten 12-inch/56 guns, roughly 11 inches of belt armour is suggested as a starting point. The classification also depends on protection across the magazines and machinery spaces, deck armour, and the degree of internal subdivision. Supporting such long, heavy guns already demands substantial structure to spread turret loads through the hull without warping it. Battleship-like subdivision combined with extensive protection would therefore move the design toward an early fast battleship. In the classification used here, Vanguard illustrates a different combination: battlecruiser internals with battleship coverage of armour. A light battleship reverses that relationship, retaining battleship internals with lighter armour coverage.