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