The checksum enables the initiator of any transaction to perform a sanity check of any specified bank account number on their own computer prior to actually initiating a request. The Extended Industry Sorting Code Directory (EISCD)
The EISCD is a comprehensive database of payments-related information for financial institutions participating in the UK Payment Schemes: Bacs, CHAPS, Faster Payments and Cheque and Credit Clearing. The BBAN, whose format is decided by the national banking community, includes the domestic bank account number, branch identifier and potentially routing information, the only restrictions being that it must be of fixed length and that only case-insensitive alphanumeric characters are used.