From Bulbapedia, the community-driven Pokémon encyclopedia.
Comparison of a box topper (right) with a regular card
A Box Topper is a special card included inside sealed booster boxes as an incentive for buying an entire box of booster packs. They were first introduced in the Legendary Collection, where they were oversized, reverse-holographic versions of cards within the set. Starting in EX Team Magma vs Team Aqua, box toppers became standard size, and not copied from other cards within their set. These standard size box toppers (along with the secret insert cards, such as Celebi ex in EX Unseen Forces) have a set number which exceeds the size of the set (e.g. "98/97") and always have a rarity. The inclusion of box toppers ceased after the release of EX Holon Phantoms, with some of the booster pack boxes containing a figurine of a Pokémon instead.
Cards with Box Topper variants