- Extensions are a key part of the FHIR specification, providing a standardized - way of placing additional data in a resource. -
-- All HAPI FHIR Resource objects support "undeclared extensions", - which are arbitrary extensions you can add. + Extensions are a key part of the FHIR specification, providing a standardized + way of placing additional data in a resource.
+ ++ The simplest way to interact with extensions (i.e. to add them to resources you are creating, or + to read them from resources you are consuming) is to treat them as "undeclared extensions". + Undeclared extensions can be added to any of the built in FHIR resource types that come with HAPI-FHIR. +
+- HAPI provides a few ways of getting data back out of - an extension value (i.e. from a received resource): + HAPI provides a few ways of accessing extension values in resources + which are receieved from other sources (i.e. downloaded by a client).