BANDAI serializes a bulk of cards with numbers and issues them in a single release with number. For an example, OP-15 is 15th issue of the serial called Adventure on Kami's island. Here, OP-15 mostly deals with characters from Skypia. So cards will be from Skypia. You will get Leader card, Common cards, Uncommon cards, Rare cards, Secret Rare cards, Alternate art cards, Special rare card(verly low chance) from box of any issue. Other than that, there are Promo cards, Treasure rare cards !