What is mean "cluster_uuid" : "_na_" in ES v2.4.1?

Elastic Stack Elasticsearch
1

Hello. I have updated ElasticSearch to v2.4.1 and now I see a new response attribute "cluster_uuid".
Why it has "na" value and what is mean?

curl localhost:9200
{
  "name" : "test1",
  "cluster_name" : "elka_test",
  "cluster_uuid" : "_na_",
  "version" : {
    "number" : "2.4.1",
    "build_hash" : "c67dc32e24162035d18d6fe1e952c4cbcbe79d16",
    "build_timestamp" : "2016-09-27T18:57:55Z",
    "build_snapshot" : false,
    "lucene_version" : "5.5.2"
  },
  "tagline" : "You Know, for Search"
}
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