This is the CodeSystem that contains all the codes in ISO-8601-Derived Periods Codes (http://terminology.hl7.org/CodeSystem/iso-8601-derived-periods) and ISO-8601-Derived Periods Codes (http://terminology.hl7.org/CodeSystem/iso-8601-derived-periods). E.g. what you have to deal with if you get resources containing codes in either of them