[[mapping-fields]] == Meta-Fields Each document has metadata associated with it, such as the `_index`, mapping <<mapping-type-field,`_type`>>, and `_id` meta-fields. The behaviour of some of these meta-fields can be customised when a mapping type is created. [float] === Identity meta-fields [horizontal] <<mapping-index-field,`_index`>>:: The index to which the document belongs. <<mapping-type-field,`_type`>>:: The document's <<mapping-type,mapping type>>. <<mapping-id-field,`_id`>>:: The document's ID. [float] === Document source meta-fields <<mapping-source-field,`_source`>>:: The original JSON representing the body of the document. {plugins}/mapper-size.html[`_size`]:: The size of the `_source` field in bytes, provided by the {plugins}/mapper-size.html[`mapper-size` plugin]. [float] === Indexing meta-fields <<mapping-field-names-field,`_field_names`>>:: All fields in the document which contain non-null values. <<mapping-ignored-field,`_ignored`>>:: All fields in the document that have been ignored at index time because of <<ignore-malformed,`ignore_malformed`>>. [float] === Routing meta-field <<mapping-routing-field,`_routing`>>:: A custom routing value which routes a document to a particular shard. [float] === Other meta-field <<mapping-meta-field,`_meta`>>:: Application specific metadata. include::fields/field-names-field.asciidoc[] include::fields/ignored-field.asciidoc[] include::fields/id-field.asciidoc[] include::fields/index-field.asciidoc[] include::fields/meta-field.asciidoc[] include::fields/routing-field.asciidoc[] include::fields/source-field.asciidoc[] include::fields/type-field.asciidoc[]