This is the ValueSet that contains all the codes in US Public Health Message Types (http://hl7.org/fhir/us/medmorph/ValueSet/us-ph-message-types) and US Public Health VaueSet - Message Types (http://hl7.org/fhir/us/ph-library/ValueSet/us-ph-valueset-message-types). E.g. what you have to deal with if you get resources containing codes in either of them