http://www.gentoo.org/proj/en/infrastructure/ldap.xml does not mention the "herd" field although I can set it (to one value) in my LDAP record.
As far as I know, it isn't used for anything anyway.
It is listed... "The following attributes were in use at some point in the past, but have been retired: gentooHerd/herd, gentooAltMail/altMail, gentooForumsUID/forumsUID."