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