This is the CodeSystem that contains all the codes in BeCSVaccineCode (https://www.ehealth.fgov.be/standards/fhir/vaccination/CodeSystem/be-cs-vaccine-code) and BeCSVaccineCode (https://www.ehealth.fgov.be/standards/fhir/vaccination/CodeSystem/be-cs-vaccine-code). E.g. what you have to deal with if you get resources containing codes in either of them