Permit explicit keys to be passed as part of a link template, to support inclusion of the "type" property. (JSONAPI)
Uniq linked documents of each type by ID, if all possess an ID.  (JSONAPI; currently automatic, not optional).
Choose the type under which linked documents are grouped by the type of the linked document, rather than the name of the relation. Uses a different declaration in schema: entity_typed or entities_typed. (To be used in combination with a matching link template). (JSONAPI)