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My understanding is that the normal convention is to use www to show that it is a web page. Some sites may use ftp. to show it is an ftp site or m. to show it is a mobile site. If you register as xoops.org though you should be able to use
http://www.xoops.org on your site and it should recognize it.
On the same server you could then have an ftp.xoops.org, m.xoops.org, doc.xoops.org dev.xoops.org and many other setups and it would still be based off the xoops.org domain name.
So you SHOULD use the www to prefix your site but you don't HAVE to.
Rodney