http://hl7.org/fhir/StructureDefinition/referencesContained|5.2.0

This indicates that the element containing this extension has content that makes reference to the specified contained resource.\n* Expression may contain CQL or FHIRPath that makes reference to ValueSets\n* string may contain FHIRPath referencing ValueSets or contain escaped XHTML referencing images (as Binaries)\n* Narrative contains 'text' which is xhtml (and can't have extensions itself) which can also refer to images\n* markdown can refer to images.