Delete-by-query Plugin sort("_doc")

Elastic Stack Elasticsearch
1

I was comparing how the delete-by-query plugin was implemented compared to how I implemented similar functionality. In the plugin I noticed the following:

            SearchSourceBuilder source = new SearchSourceBuilder()
                    .query(request.query())
                    .fields(fields)
                    .sort("_doc") // important for performance
                    .fetchSource(false)
                    .version(true);

What is the significance of sort("_doc")? The only reference I have found to _doc is in regards to scripting.