This attribute holds a unqiue identifier code the the element it is on. It is not necessary for every instance of the element to include such a code (unlike elements that include the required version of this attribute (in the alternative group of shared attributes).

This code can be used in references to the element from external systems, or from the <xref> element (or more specialised variant of it).

Defined in shared attribute group: "Common Optional ID Attributes"

This attribute is not required. It holds a unique identifier code for the element.

The DTD contains the following attribute definition (note that embedded links are to definitions within the DTD page of this guide):

                id                       ID  #IMPLIED