A query that accepts any other query as base64 encoded string.

GET /_search
{
  "query": {
    "wrapper": {
      "query": "eyJ0ZXJtIiA6IHsgInVzZXIuaWQiIDogImtpbWNoeSIgfX0=" 
    }
  }
}

This query is more useful in the context of Spring Data Elasticsearch. It’s the way a user can add custom queries when using Spring Data repositories. The user can add a @Query() annotation to a repository method. When such a method is called we do a parameter replacement in the query argument of the annotation and then send this as the query part of a search request.