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)
contact
Someone you know how to get in touch with. Often symmetric.
acquaintance
Someone who you have exchanged greetings and not much (if any) more — maybe a short conversation or two. Often symmetric.
friend
Someone you are a friend to. A compatriot, buddy, home(boy|girl) that you know. Often symmetric.
physical
met
Someone who you have actually met in person. Symmetric.
professional
co-worker
Someone a person works with, or works at the same organization as. Symmetric. Usually transitive.
colleague
Someone in the same field of study/activity. Symmetric. Often transitive.
geographical (pick at most one)
co-resident
Someone you share a street address with. Symmetric and transitive.
neighbor
Someone who lives nearby, perhaps only at an adjacent street address or doorway. Symmetric. Often transitive.
family (pick at most one)
child
A person's genetic offspring, or someone that a person has adopted and takes care of. Inverse is parent.
parent
Inverse of child.
sibling
Someone a person shares a parent with. Symmetric. Usually transitive.
spouse
Someone you are married to. Symmetric. Not transitive.
kin
A relative, someone you consider part of your extended family. Symmetric and typically transitive.
romantic
muse
Someone who brings you inspiration. No inverse.
crush
Someone you have a crush on. No inverse.
date
Someone you are dating. Symmetric. Not transitive.
sweetheart
Someone with whom you are intimate and at least somewhat committed, typically exclusively. Symmetric. Not transitive.
identity
me
A link to yourself at a different URL. Exclusive of all other XFN values. Required symmetric. There is an implicit "me" relation from the contents of a directory to the directory itself.