Existing Wording: The base definition for elements defined as part of a resource definition.

Proposed Wording: The base definition for elements defined as part of a resource definition that do not use one of the FHIR data types, but which instead define a resource-specific complex structure of child elements.

Comment:

Backbone element isn't the base definition for all elements defined as part of a resource, only those that don't use a FHIR data type

Summary:

Improve backboneelement definition