A roll number is a unique identifier for an individual's account number, normally a savings account, at a building society, states Experian. Wikipedia explains that building societies are financial institutions that offer banking services in the United Kingdom and other countries.
The roll number can also be referred to as the account number or the customer reference number, but it should not be confused with the bank account number. The number is used for direct debit and direct credit throughout the U.K. using Bacs services, notes Experian. To use the roll number with Bacs, the number has to appear in the reference field of the Bacs file and cannot exceed 18 characters.