https://forums.theregister.co.uk vs https://www.theregister.co.uk
Title says it all but still I'll say more ...
Last time I functioned as server admin
/www. = live website
/dev. = clone of live website to test security updates before applying them to /www.
/test. = clone of live website to test new features
/m.= mobile friendly version of website.
/ftp. = file repository
/forums. = forums
OK there are other options other than using subdomains ... said options either require that I create either register domainnamedeve.com or domainname.com/dev Neither saves significant amounts of space.
If they wanted to simply hide www, but display any other subdomain ... that would be acceptable. Anything else and well ... I've heard nice things about Firefox lately.