The rel="alternate" attribute along with a hreflang attribute provides access to another version of the document in a different written language.
The rel="alternate" attribute along with a media attribute provides access to another version of the document intended for a different media. This can be used to support browsers with different screen sizes, such as cell phones and other handheld devices with smaller-sized screens.
The rel="alternate" attribute along with a type attribute provides access to similar content in an alternate format. For example, this can be used to reference the syndicated feed for a blog.
<link rel="alternate feed" type="application/rss+xml" href="/blog/rss-feed.xml"/>
Although browsers should assume that the referenced content is a feed when the type attribute is recognized as a known feed format, it is probably better to include "feed" in the rel tag.