The KNAB Place Names Database will use codes corresponding to the standard ISO 3166-1 and 3166-2 (Codes for the representation of names of countries and their subdivisions), in their absence also codes of KNAB. See:
Countries | Administrative units of countries and their codes
As the codes given in ISO 3166-2 do not always cover all the necessary administrative units and tend to become outdated, there are also KNAB's own codes in use. In order to distinguish KNAB's codes from those of ISO one needs to know that the ISO standard denotes administrative units always with the help of numbers (1, 01, XII) or one to three capital letters (A, BH, LAN). The codes follow where possible national patterns and preferences. The KNAB codes contain both uppercase and lowercase letters: Kb, Can, etc. Whereas ISO uses only the basic Latin characters (ASCII), the KNAB codes may also contain letters with diacritical marks, in accordance with the spelling of the name in question.
The only exceptions to the given rules of distinction are the province codes for Finland (coded by numbers in KNAB, according to national practices) that are absent in ISO 3166-2, and the codes for administrative units in Estonia (in ISO these are number codes, as officially decreed).
Due to technical reasons some of the country codes within the KNAB original database were given using the three-letter codes of ISO 3166-1. Those were HRV for Croatia (normally HR), LVA for Latvia (LV), SAU for Saudi Arabia (SA) and VAT for Vatican (VA). The reason for doing so was to avoid confusion with the appropriate county abbreviations for Estonia. While these codes have been replaced by the usual 2-letter codes in the Internet-version, there are instances (e.g. in source abbreviations) where the three-letter codes can also be detected.