In addition to the IBAN check digits, many countries have their own national check digits used within the BBAN, as part of their national account number formats. Each country determines its own algorithm used for assigning and validating the national check digits - some relying on international standards, some inventing their own national standard, and some allowing each bank to decide if or how to implement them. Some algorithms apply to the entire BBAN, and others to one or more of the fields within it. The check digits may be considered an integral part of the account number, or an external field separate from the account number, depending on the country's rules.