As already been pointed out in FHIR-19250 and FHIR-19249, the boundaries between DocumentReference and DiagnosticReport are somewhat unsharp. In fact, both resources match with their definition to a large set of possible documents. For implementers it is hard to distinguish the right resource, ending up in recommendations as given in the US Core Clinical Notes Guidance:

In order to enable consistent access to scanned narrative-only clinical reports the Argonaut Clinical Note Server SHALL expose these reports through both DiagnosticReport and DocumentReference [...]

Following this recommendation, duplicate resources link to the very same content, to make sure everyone finds all relevant documents in a server. Then at least those two resources should refer to each other. In the current build of DiagnosticReport (as of 2019-11-22) the DiagnosticReport.media.link has been made a DocumentReference (unlike R4 release).
In the resource DocumentReference on the other hand, only DocumentReference.context.related could be used to link a DiagnosticReport.
This is not self-explanatory at all and a compatible implementation won't be assured among all implementers.

Please clarify these specific use cases of having both resources for one single document and describe a designated way of linking both to each other.