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See the executive summary, the developer's introduction, clinical introduction, or architect's introduction, and then the FHIR overview / roadmap & Timelines. See also the open license (and don't miss the full Table of Contents or you can search this specification).
Level 1 Basic framework on which the specification is build
Level 2 Supporting Implementation, and binding to external specifications
Level 3 Linking to real world concepts in the healthcare system
Level 4 Record-keeping and Data Exchange for the healthcare process
Level 5 Providing the ability to reason about the healthcare process
Decision Support, Clinical Quality Measures
Specifications based on the FHIR standard
Enabling health interoperability through FHIR
Note that translations are not always up to date
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