Whilst most standard UK bank accounts have an 8 digit account number and 6 digit sort code, some Building Society accounts may also have what's referred to as a 'Building Society roll number' or just a 'roll number' - a reference code with letters and numbers.
If you are making a payment to a Building Society account, then the Building Society should provide you with the roll number if required. When making a payment online, you should add the roll number as the reference of the payment you are sending.
Though it’s difficult to say how many financial institutions still use roll numbers to uniquely identify an individual’s account (usually a savings account), customers will sometimes be asked to contact their bank or Building Dociety to obtain the eight-digit account number and the six-digit sort code equivalent if say, setting up a direct debit thus negating the use of the roll number.