Indicates the unique identifier for an associated element, so that the element’s content may be cross-referenced. IDs must be unique inside their document (not just within their element type), and the XML parser enforces this.
The ID and IDREF attribute values constitute a primitive reference mechanism for XML. For example, when an id attribute has been used on a <page-start> element, that identifier can be referenced by an IDREF-style attribute on a cross-reference <xref> element, so the page start can be pointed to.
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.)