IBANs, cards, balances - how to manage all of this?

Once you are starting a payment account and/or card issuing project you need to learn key definitions and relations between those various parameters.

Balance ID - this is a real "account" in the Verestro system. This number is connected with User ID and means that the user has an account and balance in our system. The user can keep money on this Balance ID.  Of course, one user can have multiple balances but a single balance can belong to one user only

IBAN - this number is often mixed with Balance ID.  IBAN is a number through which the user can receive money to his balance via wire transfer. IBAN is not a balance ID. Generally it does not make sense to have more than one IBAN for one balance. Normally you issue one IBAN for one balance. Usually a user can have more IBANs and balances if he wants to keep money on separate accounts, in various currencies etc.

Card number - easier to understand, just a card number issued to a particular balance ID (not to IBAN!). A user can have multiple cards connected to one balance ID.

Once preparing to project with Verestro, please learn the above definitions. More info here: https://developer.verestro.com/shelves/card-issuing-ibans