id is the identifier of the topic or location that is being cross-referenced.
Cross-references are translated into chapter or section titles, heads, figure captions, list items, or line numbers. The cross-reference text becomes a hyperlink that, when chosen by a user, jumps to the cross-referenced location.
To create a cross-reference, an id must be defined in the element that you intend to refer to. You use the id of the destination element as the id parameter in the <xref> tag. This creates a hyperlink from the <xref> element to the destination element. The id must be spelled exactly the same. Capitalization, however, is not significant.
The id parameter can appear with:
<chapter> <s1>, <s2>,...<s9> <otherfront> <p> <image> <item> <figure> <location> <rsect>
A cross-reference to an id that contains an underscore (such as "_abstract" or "_hometopic") is not allowed. Instead, use the <link> element.