The definition of CarePlan.participant.role imposes no constraints at all on its value. Other participant role elements are all defined in terms of value sets that obtain their values from http://hl7.org/fhir/v3/RoleCode (e.g. RelatedPerson.relationship). A value set for this element (e.g. http://hl7.org/fhir/vs/carePlanParticipantRole) should be defined, including a sufficient broad set of role codes to accommodate most uses.