This is the CodeSystem that contains all the codes in PSS-QSI procedure codes (https://www.ehealth.fgov.be/standards/fhir/pss/CodeSystem/PSSQSIProcedures) and PSS-QSI procedure codes (https://www.ehealth.fgov.be/standards/fhir/pss/CodeSystem/PSSQSIProcedures). E.g. what you have to deal with if you get resources containing codes in either of them