This is the CodeSystem that contains all the codes in STARS Message Type (https://starsapi.csiro.au/fhir-ig/stars/CodeSystem/message-type-code-system) and STARS Message Type (https://starsapi.csiro.au/fhir-ig/stars/CodeSystem/message-type-code-system). E.g. what you have to deal with if you get resources containing codes in either of them