The draft version of ISO 3166 was based on the US standard FIPS 10-4, which assigns different numbers to the provinces.
In 2005, the current version of the standard was published, introducing the codes for Amnat Charoen, Nong Bua Lam Phu and Sa Kaeo, created in 1993.
Since the last publication of the standard, the province Bueng Kan and the district Galyani Vadhana as well as some subdistricts in Bangkok were created.
The municipalities listed in the standard also have four digit codes like the districts, however starting with 99 and counting down.
For several provinces the number of municipalities is now so high that no longer all can receive a code within the scheme.