Unique internal identifier of an element; allows element to be cross-referenced (made into a link). Value must be unique across a document, not just for an element type.
The ID and IDREF attribute values form a primitive reference mechanism for XML that allow internal linking, for example, from a reference in the text to a figure or a table. As another example, when an @id attribute has been used on a <glyph-data> element, that identifier can be referenced by an IDREF-style attribute on a <glyph-ref> element, so that the full glyph data can be pointed to and need not be repeated every time the character is used.
An identifier string is typically 1-32 characters in length and must start with a letter of the alphabet. (It is an XML rule that identifiers may not start with a digit.)
External Identifiers: This attribute holds an internal document identifier that can be used by software to perform a simple link. It should not be confused with elements such as <book-id>, <contrib-id>, and <institution-id> that are used to hold externally defined identifiers such as a DOI (for the book id), an ORCID personal identifier (for the contributor id), or a Ringgold identifier (for an institution id).
| Value | Meaning |
|---|---|
| An XML identifier (ID) | A unique identifier for the element. An XML parser can verify that an IDREF-style attribute pointing to one of these is pointing to a valid ID. |
| Restriction: | |
| Value | Meaning |
|---|---|
| An XML identifier (ID) | A unique identifier for the element. An XML parser can verify that an IDREF-style attribute pointing to one of these is pointing to a valid ID. |
| Restriction: | |