My hunch is that the idea that some banks have a mainframe user account per bank customer is a red herring, and that the real answer is that the customers are i

My hunch is that the idea that some banks have a mainframe user account per bank customer is a red herring, and that the real answer is that the customers are indeed in a separate database table. But that database is a mainframe database, and the field is a fixed width that maybe could be changed but the mainframe admins see no reason to.