Well, it's a number in a ledger system somewhere (e.g. mainframe system, Oracle database, or what have you), with a lot of auditing, reporting, and procedures a

Well, it's a number in a ledger system somewhere (e.g. mainframe system, Oracle database, or what have you), with a lot of auditing, reporting, and procedures around changing that number. (Though, having an internal API to instantly get a new bank account number always seemed kinda funny to me). Slightly related, I'd have to find it again, but I recall there was at some point an assembly level hack on some Oracle database holding core banking in one country.