This is the host setup to receive Squid’s announcement messages. The default value, tracker.ircache.net is the only server I know about. You can search the tracker.ircache.net database by visiting http://www.ircache.net/Tracker/.
Note that if you set cache_mgr, your email address may be available to random people. On more than one occasion I have seen commercial caching vendors target Squid users by collecting their email addresses from this database.
announce_period, announce_file, announce_port