The FHIR Tooling Extensions IG (hl7.fhir.uv.tools) is not a patient-facing clinical guide. It is the extensions, code systems, and logical models used inside the FHIR toolchain — the official validator, code generators, terminology tooling, and the IG Publisher. FHIR Infrastructure WG maintains it continuously (CC0, not balloted) for people who build or rely on those tools.

When our validator workbench behaves a certain way — snapshot generation, additional bindings, IG parameters — this IG is usually where that behavior is specified. It also defines CDS Hooks logical models and matchetypes: template resources that assert "this payload should look like this," a standards-native pattern for scenario expectations.

What ImOnFHIR does with it: we point at it as the authoritative reference for tooling behavior. We do not implement the tooling extensions ourselves; we recognize them when IGs or payloads contain them, and we watch matchetypes / ig-parameter docs before inventing our own assertion format for test scenarios.