Roster options question

"Paul C." <PaulC-iJrn4LV3OmpWk0Htik3J/[email protected]> Fri, 04 Mar 2005 17:24:18 -0500
Newsgroups gmane.network.jabber.exodus.user
Organization Car-Part.com
Message-ID <[email protected]>
under tools, options, roster - what is the "detect contacts which are 
unreachable or no longer exist"?  How does this work and in what cases?


Thanks
Paul