Key terms you need to know from the medical and regulatory lingo archives
21st Century Cures
Act of Congress that mandated having APIs as part of a certified electronic health record in the United States. Also defined and penalized "information blocking" of "electronic health information."
Electronic Health Information (or EHI) is a term more broadly defined then ePHI covered in HIPAA and includes cost information for patients.
The U.S. Core Data for Interoperability (or USCDI) defines which data represents "all electronic health information" for purposes of 21st Century Cures. That is, USCDI defines what is available in the FHIR APIs for any given EHR vendor.