From WHATWG Wiki

The "service" Link Relation

The "service" link relation is used to reference resources that describe a service interface (e.g. an Atom Publishing Protocol endpoint). The type attribute indicates the kind of service description being referenced. The "service" link is used by user agents as a means of automatically discovering service descriptions and performing basic configuration tasks.

For example, to reference an Atom Publishing Protocol Service Document, the type attribute would be "application/atomsvc+xml".

  <link rel="service" type="application/atomsvc+xml" href="http://www.example.org/atomsvc.xml" />

By convention, if a document contains multiple references to alternative service descriptions, a class="preferred" attribute can be used to indicate which of the service descriptions is considered to be the preferred choice.

  <link rel="service" class="preferred" type="application/atomsvc+xml" href="http://www.example.org/atomsvc.xml" />

More information: http://jcheng.wordpress.com/2007/10/16/how-wlw-speaks-atompub-part-1-autodiscovery/