the intent is as written...   xor  is only  true when either: 1 name and/or family is present  or 2 the element is absent and indicated with a DAR extension. not both however the invariant is incorrect as written.  (the  name context is missing from the DAR part).  Change context to name and rewrite the expression as    ~~~ (family.exists() or given.exists()) xor extension.where(url='http://hl7.org/fhir/StructureDefinition/data-absent-reason').exists() ~~~   ( and update the XPath to new context) Will rewrite the text description to be clearer: "At least Patient.name.given and/or Patient.name.family are present or no data is available and only the Data Absent Reason Extension is present."