This is the ValueSet that contains all the codes in ObservationMeasureCountableItems (http://terminology.hl7.org/ValueSet/v3-ObservationMeasureCountableItems) and ObservationMeasureCountableItems (http://terminology.hl7.org/ValueSet/v3-ObservationMeasureCountableItems). E.g. what you have to deal with if you get resources containing codes in either of them