The first one or two digits identify the bank with the rest of the sortode identifying the branch an ccount is held.
IBAN and BIC codes are widely used means of identifying bank account numbers, but not the only international system.
Although there is a strong correlation between BIC Codes and sort codes, sort codes are not explicitly encoded into BIC codes.
In particular cases where 00 is a valid check digit, 97 will not be a valid check digit, likewise if 01 is a valid check digit, 98 will not be a valid check digit, similarly with 02 and 99.
This standard was revised in 2003 and again in 2007 when it was split into two parts.