http://hl7.org/fhir/StructureDefinition/elementdefinition-profile-element|4.0.1

The specific element to use in the referenced profile. This is used when a backbone element is being profiled, rather than an established type.

element_id	types
Extension
Extension.extension
Extension.url
Extension.value[x]	string
Extension
Extension.id	http://hl7.org/fhirpath/System.String
Extension.extension	Extension
Extension.url	http://hl7.org/fhirpath/System.String
Extension.value[x]	string