elasticsearch/docs/reference/mapping/fields/uid-field.asciidoc

[[mapping-uid-field]]
=== `_uid`

Each document indexed is associated with an id and a type, the internal
`_uid` field is the unique identifier of a document within an index and
is composed of the type and the id (meaning that different types can
have the same id and still maintain uniqueness).

The `_uid` field is automatically used when `_type` is not indexed to
perform type based filtering, and does not require the `_id` to be
indexed.