This field contains the bank key under which the bank data is stored for each country. The country-specific meaning of the bank key is determined when the country code is defined.

Banks usually operate under their bank sort code. The bank sort code appears again in the control data for the bank. In some countries, the bank account number performs this function. In this case, there is no bank sort code; the bank data is stored under the account number.

For data medium exchange, it can also be useful to be able to enter banks for foreign business partners without a bank sort code, even for countries that use them. In these cases, the bank key can be assigned internally. If the bank data is to be stored under a different key (such as a SWIFT code, for example), it is possible to arrange for external number assignment.