The "role" element was in FHIR STU3.
http://hl7.org/fhir/STU3/encounter-definitions.html#Encounter.diagnosis.role
FHIR R4 has the "use" element.
http://hl7.org/fhir/encounter-definitions.html#Encounter.diagnosis.use

{{Change the example to }}

" ... (Encounter.diagnosis D where D.use ~ ToConcept("Billing") ..."