A type of span normally used to create a hypertext link around a piece of text.
In HTML, the a tag was required whenever a hypertext link needed to be placed, and could be used around text or images. In this packages XML syntax, a hypertext is signified by specifying the href attribute. This can be on any element, not just an a, as you can see in the examples.
The a element is here mainly because it's familiar to users coming from HTML. From a CSS2 point of view however, it's identical to the u element.
To define a hypertext link, use the <A> tag with an href attribute to indicate the start of the hypertext link, and use the </A> tag to indicate the end of the link. When the user clicks any content between the <a href> and </A> tags, the link is activated.
<a href="#Chapter1">Go to Chapter 1</a> is identical to <span text-decoration="underline" href="#Chapter1">Go to Chapter 1</a>