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.
typedestination
baseDefinitionhttp://hl7.org/fhir/StructureDefinition/Extension
typehttp://hl7.org/fhir/StructureDefinition/Extension
Extension.value[x].typehttp://hl7.org/fhir/StructureDefinition/string