This is the ValueSet that contains all the codes in USPS Two Letter Alphabetic Codes (http://hl7.org/fhir/us/core/ValueSet/us-core-usps-state) and USPS Two Letter Alphabetic Codes (http://hl7.org/fhir/us/core/ValueSet/us-core-usps-state). E.g. what you have to deal with if you get resources containing codes in either of them