The same content may be served by multiple means: http, ftp, and rsync are common examples. Content may also be served via a 'private' URL only visible to other mirror admins. Such private URLs usually include a nonstandard rsync port, and/or a username and password. Admins from Mirror Sites on your SiteToSite list can see these private URLs.
[Add]Note: This HostCategory is marked always_up2date. It is believed to carry the complete content of the master under this category, and is always in sync with the master. Consult a sysadmin if this is incorrect.