In HTML 4.01, there is the profile attribute for the head element:
This attribute specifies the location of one or more meta data profiles, separated by white space. For future extensions, user agents should consider the value to be a list even though this specification only considers the first URI to be significant.
It is used to link to meta data profiles.
In HTML5, the profile attribute was dropped.
As an alternative to it, the rel value profile could be used (with several advantages over the profile attribute). RFC 6906 describes the "profile Link Relation Type".
An example format for such meta data profiles (whether for the profile attribute or the rel value) is XMDP: XHTML Meta Data Profiles. It is defined at http://gmpg.org/xmdp/:
a simple XHTML-based format for defining HTML meta data profiles easy to read and write by both humans and machines
In your example, the profile for XFN is used, the Xhtml Friends Network (archive.org link):
a simple way to represent human relationships using hyperlinks
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