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.

I worked at Sears selling TVs while in college from 2002-04, and even in their latest POS systems anyone could walk up to the thermal printer, press a button (even with the register itself locked) and print out a reverse-chronological “journal roll,” which included names, addresses, phone numbers and full credit card numbers and expiration dates for every transaction. Crazy that anyone thought that was OK in the early 2000s.