A bet builder is different from an accumulator because of the ‘related contingencies’.
Bet builders provide the same game multipliers whereas an accumulator will not allow outcomes of the same game in a football acca.
Wincasts and score casts are allowed, but the odds are shortened from traditional double bet on both selections (as if the first goalscorer happens, this hugely affects the price of the win bet).
Bet builders allow you to add multiple selections to one custom bet, adjusting the betting odds accordingly.