"Extra Boosters" (エクストラブースター Ekusutora Būsutā) are packs of cards that are used to build or edit decks with new cards, but are separate from the standard Booster Sets. Each pack contains 5 random cards out of 48 different cards available.
The ratio of rarity is divided into 1 BR/2 RRR/5 RR/10 R/10 UC/20 C, in addition to this, 6 of them are also SP. The Buddy Rare (BR) is a unique card that also will come in the next set.
As of June 5, 2015, this series is replaced by the "H (Hundred) Booster Set" series in the Japanese format, and as of June 6 in the English format. Both of them add Secret rare to ratio of rarity, as well as add an additional R in each pack.