YGO TCG Structure Deck: Hero Strike – Rules of Play says:

Elemental HERO Decks rely on the power of Fusion Summoning, allowing you to fuse monsters together with Spell Cards like Polymerization and Miracle Fusion to Summon Elemental HERO Fusion Monsters; in the HERO Strike Structure Deck, brand new Spell Cards are introduced to give your monsters a new secret identity as a Masked HERO monster as well! As a special bonus, the HERO Strike Structure Deck also includes one new Ultra Rare Card and four new Super Rare Cards, bringing the total count to 45 Cards!