A formal specification defining how clinical health data in a national or system-specific format is transformed into the EU-standard EEHRxF/FHIR format required for cross-border exchange via MyHealth@EU. A Semantic Mapping Rule governs both structural transformation (which FHIR resource types to produce) and semantic translation (which terminology codes to use), and is technically executed by the HAPI FHIR StructureMap engine. It is part of the semantic traceability chain Local clinical dataset - Semantic Mapping Rule → Normalised Health Dataset → Digital Twin Input Feature Set.