http://hl7.org/fhir/StructureDefinition/structuredefinition-explicit-type-name|5.2.0

A name to use for the type, in implementations. This is a suggestion; it's not a normative part of the FHIR specification, but it does appear in the UML diagrams, and is used in generated code, schemas, etc.to identify the type.
typedestination
baseDefinitionhttp://hl7.org/fhir/StructureDefinition/Extension
typehttp://hl7.org/fhir/StructureDefinition/Extension
Extension.value[x].typehttp://hl7.org/fhir/StructureDefinition/string