This article is from the rec.audio.* FAQ, by with numerous contributions by Bob Neidorff others.
Cross posting means sending the message/posting to more than
one newsgroup. This is more efficient than posting the same
message twice, as it stores less space on disks, takes less
time to transmit, etc. If you need to address the message to
more than one newsgroup, put all newsgroups in the Newsgroup:
However, there is rarely a reason to cross-post to many of
the rec.audio.x newsgroups. If you have a repair question,
it belongs in rec.audio.tech and no where else. If you are
selling a tuner, post to rec.audio.marketplace. Want opinions
on the right cable? Post to rec.audio.opinion. Not sure?
Post to rec.audio.misc.