battle spirits booster box TCG - [BS-66] The Contract Saga: Kai Vol.3: Convene Boo but also reduce your opponents life by two with a single attack AmiAmi [Character & Hobby Shop]

That is the probability that a card will appear in a particular or special event and if it has unique attributes, such as the stock of cards or the Pokmon type

While it is typical that the more powerful the monster the more requirements it will have, for "roid" monsters, this factor is much more extreme

Once per turn, during the Standby Phase, if this card is in the GY because it was sent there last turn: You can target 1 Level 7 or 8 Dragon monster in your GY, except "Arkbrave Dragon"

This is the kind of card control players always want to have

The European versions of the game did not come with any promotional cards