Health systems can technically include multiple attachments as part of a clinical note. Our assumption is that each of those attachments should be the same note (perhaps in different formats like PDF or HTML), and that they shouldn't include multiple different notes in a single DocRef (for lots of reasons, but primarily safety).

The current version of US Core (and FHIR R4/B) do not make this clear. For example, from the [R4 base spec|https://hl7.org/fhir/R4/documentreference-definitions.html]:

The document and format referenced. There may be multiple content element repetitions, each with a different format.

After speaking with Eric Haas, he confirmed that this is the intent, and the language in the [base spec for R5|https://hl7.org/fhir/R5/documentreference-definitions.html] has been strengthened to reflect this: 

The document and format referenced. If there are multiple content element repetitions, these must all represent the same document in different format, or attachment metadata.

Given the prevalence of FHIR R4/B via adoption of US Core, this base spec should also be updated with the same/similar language. The [US Core implementation guidance for DocumentReference|http://hl7.org/fhir/us/core/StructureDefinition-us-core-documentreference.html] should also be updated to make this clear.