"Linkage" refers to the representation of links (i.e. associations) between resources.

In HAL, links are represented by embedding child documents in parent documents.

In JSON API, links are represented by listing the `id(s)`, `type(s)`, and/or `href(s)` of related resources in the `links` hash of a document. These related resources are now (optionally) represented in the `linked` hash at the top level of compound documents (the text should be changed from saying "JSON API flattens the entire graph of objects at the top level").