It contains the International Bank Account Number (IBAN) according ISO 13616. It consists of an ISO 3166-1 alpha-2 country code, followed by two check digits that are calculated using a mod-97 technique and Basic Bank Account Number (BBAN) with up to 30 alphanumeric characters. There are no spaces inside (spaces are added to the printed form only for better readability). For the Czech Republic IBAN has the format CZkkbbbbrrrrrruuuuuuuuuu, where kk are check digits, bbbb is a 4-digits bank code, rrrrrr is the first part and uuuuuuuuuu is the second part of an account number (all with leading zeroes).
Currently not used.