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