Person isn't allowed to be referenced as a participant in FHIR anywhere.  Person is only allowed to be used to link together multiple instances of Patient, Practitioner and/or RelatedPerson.  Every human being (or animal) that is a participant in a FHIR resource MUST be conveyed using either Patient, Practitioner or RelatedPerson.  Someone who's tracking their steps using a FitBit is a 'Patient' in FHIR terms, even if they've never seen - and will never see - a healthcare practitioner.