Additional Resources is a formal extensibility mechanism introduced in FHIR R6 that allows new resource types to be defined outside the core FHIR specification while still functioning as first-class FHIR resources. They are defined in Implementation Guides (called "Incubator IGs"), published on their own release cycles, and are expected to eventually migrate back into the core specification once stable.
The mechanism exists because R6 is the first fully normative FHIR release — once published, no breaking changes are allowed. Resources that are still immature (FMM 0-2) cannot be frozen normatively, so they need an alternative development pathway.
"Our market feedback is very strong that further change to the FHIR resources is getting very expensive, and one of the few incentives for the market to move to R6 is stability." — Grahame Grieve, FMG communication to committees (Aug 2025)