Las etiquetas pueden contener atributos:

<div type="book" xml:id="la.3" n="3">contents</div>

The main part is the tag itself (<div>), which must always go at the beginning of the angle brackets. But it can also contain within it one or more attributes, which follow it (@type, @n, @xml:id, @xml:lang, @target, etc.), and which indicate, between commas, the value that the attribute has.

The values, depending on the attribute, can be free or limited to a series of possibilities. For example, the value of a number, @n, will necessarily be different in each tag; whereas the value of @type of an <div> must belong to a predetermined list, and can repeat itself ("edition", "translation", "book", etc.). 

Many attributes can appear associated with different tags. Others, though, can only be added to a specific tag.