eHealth Infrastructure (v2019.5.1) 

{
  "resourceType" : "SearchParameter",
  "id" : "ehealth-communication-search-recipientIdentifier",
  "text" : {
    "status" : "generated",
    "div" : "<div xmlns=\"http://www.w3.org/1999/xhtml\"><p><b>Generated Narrative with Details</b></p><p><b>id</b>: ehealth-communication-search-recipientIdentifier</p><p><b>url</b>: <a href=\"http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/recipientIdentifier\">http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/recipientIdentifier</a></p><p><b>name</b>: recipientIdentifier</p><p><b>status</b>: active</p><p><b>code</b>: recipientIdentifier</p><p><b>base</b>: Communication</p><p><b>type</b>: token</p><p><b>description</b>: Search parameter for finding communication based on the identifier(s) of the recipient</p><p><b>expression</b>: Communication.recipient.identifier</p><p><b>xpathUsage</b>: normal</p></div>"
  },
  "url" : "http://ehealth.sundhed.dk/fhir/SearchParameter/Communication/recipientIdentifier",
  "name" : "recipientIdentifier",
  "status" : "active",
  "code" : "recipientIdentifier",
  "base" : [
    "Communication"
  ],
  "type" : "token",
  "description" : "Search parameter for finding communication based on the identifier(s) of the recipient",
  "expression" : "Communication.recipient.identifier",
  "xpathUsage" : "normal"
}