This is the ValueSet that contains all the codes in US Public Health Message Significance Category (http://hl7.org/fhir/us/ecr/ValueSet/us-ph-message-significance-category) and US Public Health Message Significance Category (http://hl7.org/fhir/us/ecr/ValueSet/us-ph-message-significance-category). E.g. what you have to deal with if you get resources containing codes in either of them