The so called NUCC code system (NUCC Provider Taxonomy) inside of FHIR US Core, where NUCC is used, is improperly defined. You can’t go to an html page and see the code system because the canonical url referenced is not in rendered, but you can see the codes in the USCoreProviderSpecialityNucc value set. At issue is the made up codes that start with the phrase “base-“; these do not exist in the code system and need to be removed from FHIR rendering of the code system.