I skimmed through FAQs and didn't see any documentation on this feature, not even a pointer to a search engine... It sounds like it should go in Syn though.
rho mentioned that there's a little bit on the bots page.
"FOAF: A user's information page using the Friend of a Friend XML format. Available at the URL http://www.livejournal.com/users/username/data/foaf"
What is FOAF? Why should I use it?
FOAF, an acronym for "Friend of a Friend", is a way to standardize Interests data, Friends data, and other information about people. It will not replace your User Info page, but allows users to use the same, or similar, information across different web sites, rather than retyping it. FOAF can apply to any account type, not just Paid accounts.
For more information on FOAF please see the ljfoaf community referenced below or search for FOAF with your favorite search engine.
Please note that the above links are unofficial LiveJournal resources or are off-site. As such, LiveJournal Support Volunteers can not provide further assistance with their content.