Formidable was the second Illustrious-class carrier, a design commonly called an armoured carrier. The flight deck was protected over a defined area, and the hangar sat within an armoured box; much of the hull had lighter protection. The strength deck and protected hangar concentrated the survival system around flight operations. That distinction matters when judging the design's purpose and Formidable's later battle damage.
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'Armoured carrier' as shorthand for a protected hangar
Formidable's flight-deck armour covered a defined area, and the protected hangar did not make the whole Illustrious-class carrier armoured.