There are four metadata fields associated with document identity:

_id

The string ID of the document

_type

The type name of the document

_index

The index where the document lives

_uid

The _type and _id concatenated together as type#id

By default, the _uid field is stored (can be retrieved) and indexed (searchable). The _type field is indexed but not stored, and the _id and _index fields are neither indexed nor stored, meaning they don’t really exist.

In spite of this, you can query the _id field as though it were a real field. Elasticsearch uses the _uid field to derive the _id. Although you can change the index and store settings for these fields, you almost never need to do so.