This is a read-only, R4 FHIR API which aggregates data from all input routes, both FHIR and non-FHIR. The aggregation will perform STU3->R4 mappings as required. PKB will iteratively build support for resources, beginning with Appointments. Complex access control applies since this contains data from multiple sources.
Please see the "rest" section of the corresponding CapabilityStatement for details of which endpoints are supported:
http://fhir.patientsknowbest.com/capabilitystatement/aggregator
Please see the Aggregated Roadmap page for planned changes to this endpoint.