http://hl7.org/fhir/StructureDefinition/elementdefinition-profile-element|5.2.0

The specific element to use in the referenced profile, identified by the ElementDefinition.id. This is used when a backbone element is being profiled, rather than an established type.
element_idtypes
Extension
Extension.extension
Extension.url
Extension.value[x]
Extension
Extension.id
Extension.extension
Extension.url
Extension.value[x]