http://hl7.org/fhir/StructureDefinition/elementdefinition-bestpractice-explanation|4.0.1

Explains why an invariant is labelled as a best practice invariant.

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