This is the ValueSet that contains all the codes in US Core Provider Speciality (NUCC) (http://hl7.org/fhir/us/core/ValueSet/us-core-provider-specialty) and Healthcare Provider Taxonomy (http://cts.nlm.nih.gov/fhir/ValueSet/2.16.840.1.114222.4.11.1066). E.g. what you have to deal with if you get resources containing codes in either of them