http://hl7.org/fhir/StructureDefinition/elementdefinition-bestpractice-explanation|5.2.0

Explains why an invariant is labelled as a best practice invariant.
typedestination
baseDefinitionhttp://hl7.org/fhir/StructureDefinition/Extension
typehttp://hl7.org/fhir/StructureDefinition/Extension
Extension.value[x].typehttp://hl7.org/fhir/StructureDefinition/markdown