What is meant by the rel="bookmark" link attribute?

What is the purpose of the rel="bookmark" attribute in <a> tags? For example:

<a href="http://stackoverflow.com/questions/ask" rel="bookmark">Click Here</a>

Does it serve any SEO- or SEM-related purpose?

This has no SEO value or purpose. I believe the rel=bookmark tag was intended to mark permalinks but it never really gained traction

An important use of the LINK element is to define a toolbar of navigation buttons or an equivalent mechanism such as menu items.

LINK relationship values reserved for toolbars are:

REL=Home The link references a home page or the top of some hierarchy.

REL=ToC The link references a document serving as a table of contents.

REL=Index The link references a document providing an index for the current document.

REL=Glossary The link references a document providing a glossary of terms that pertain to the current document.

REL=Copyright The link references a copyright statement for the current document.

REL=Up When the document forms part of a hierarchy, this link references the immediate parent of the current document.

REL=Next The link references the next document to visit in a guided tour.

REL=Previous The link references the previous document in a guided tour.

REL=Help The link references a document offering help, e.g. describing the wider context and offering further links to relevant documents. This is aimed at reorienting users who have lost their way.

REL=Bookmark Bookmarks are used to provide direct links to key entry points into an extended document. The TITLE attribute may be used to label the bookmark. Several bookmarks may be defined in each document, and provide a means for orienting users in extended documents.

From: http://www.w3.org/MarkUp/html3/dochead.html

The attribute "rel" means relationship. It's a microformats. The HTML4 spec describes a bookmark as "a link to a key entry point within an extended document". By convention (citation needed), this entry point also captures the notion of a "permalink". Exemple:

<a href="archive/entry.html" rel="bookmark">A Document Entry</a>

Another exemple of rel attribute for tags:

<a href="http://technorati.com/tag/tech" rel="tag">tech</a>

http://microformats.org/wiki/rel-bookmark#rel.3D.22bookmark.22