As described in HTML4 Meta data profiles.
HTML4 definition of the ‘rel’ attribute. Here are some additional values, each of which can be used or omitted in any combination (unless otherwise noted, and except where prohibited by law) and their meanings, symmetry, transitivity and inverse if any. Please see the XFN home page for more information about XFN.
friendship (pick at most one)
- Someone you know how to get in touch with. Often symmetric.
- Someone who you have exchanged greetings and not much (if any) more — maybe a short conversation or two. Often symmetric.
- Someone you are a friend to. A compatriot, buddy, home(boy|girl) that you know. Often symmetric.
- Someone who you have actually met in person. Symmetric.
- Someone a person works with, or works at the same organization as. Symmetric. Usually transitive.