This was a checksum that machines could run locally, to make sure the account # was "valid". Then, in batch, systems would connect to the bank for the account

This was a checksum that machines could run locally, to make sure the account # was "valid". Then, in batch, systems would connect to the bank for the account interaction. Some services (AOL, when it charged by the minute) wouldnt do the actual bank reconciliation for a few days, during which you could use the service.