API Documentation for the HDCM, HIP and HIU are available from the source code repositories.
The documentations are maintained in OpenAPI Specification (OAS), commonly known as swagger API documentation format. To view the documentation on the browser, you may follow the following steps.
Many of the contracts have errors and meta codes. Which are documented here
Health Information is exchanged via FHIR bundles. FHIR is a standard for health care data exchange, published by HL7. The intent of using FHIR is towards standardization of schema, i.e machine readable, rather than addressing or enforcing semantic standards for interoperabilty. See here for health data representation in FHIR in the reference implementation stack.