The cda-core-2.0 logical models (https://github.com/HL7/cda-core-2.0) use an "elementdefinition-defaulttype" extension on certain elements that are the type can't be known based on the CDA instance or the StructureDefinition (See SubstanceAdministration.effectiveTime for example). The used canonical is "http://hl7.org/fhir/StructureDefinition/elementdefinition-defaulttype". However it is currently not listed on the official core extension registry. Proposal to add it as a core extension as there is value for other logical models as well where the type is represented by ElementDefinition.typeAttr.