It is a term from blackjack denoting the standard position rules available in most varieties of the game.
Double – the ability to double the bet in blackjack.
The rule applies in most varieties of the game but may vary in nuances. For example, it often happens that a double is allowed only on nine, ten or eleven points.
Usually, you can double your bet only on the first two cards. Whether it is possible to type further cards on Boxing, too depends on the rules of this blackjack version.