The suffix is automatically allocated by Voca, and are numbered in sequence starting with 01 for the first subbranch. For example, sorting code 00-00-00 would represent the main bank office, and the first subbank office registered would have the suffix 01.
The rules vary to a great extent between the countries. 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.
Although there is a strong correlation between BIC Codes and sort codes, sort codes are not explicitly encoded into BIC codes.