I am still trying to figure out what does \_doc represents in elasticsearch?
From the documentation, two places I could find it's usage are:

1. While in sorting, it is recommended that \_doc is better because the benefit of sorting by \_doc is that elasticsearch can efficiently skip to the next matching document when moving to the next page (it will simply ignore all docs that have a smaller doc id than the last returned document). [Source](https://github.com/elastic/elasticsearch/pull/12983#issuecomment-181118167)
2. Another reference to \_doc was mentioned in [this](https://github.com/elastic/elasticsearch/issues/4492) git request which talks putting field name against \_doc.

Can someone exactly tell what is \_doc actually?