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

Mark that an invariant represents 'best practice' rule - a rule that implementers may choose to enforce at error level in some or all circumstances.
element_idtypes
Extension
Extension.extension
Extension.url
Extension.value[x]
Extension
Extension.id
Extension.extension
Extension.url
Extension.value[x]