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So long as the host record points at the correct UH server it makes diddly squat difference for any of the services except HTTP and HTTPS. All the logins are
user@domain.com or user#domain.com so you always know which site someone is related to.
The IP address can also be either a dedicated or shared IP, it makes no difference so long as it is the same UH server. A site with a dedicated IP could still use the shared IP for non-HTTP[S] services. When a site has a dedicated IP address you could point whatever domain you liked at that IP and the same site would be returned (unless you did some .htaccess trickery). This is because the IP address is the identifier in this instance.
The host name used only becomes key for HTTP sites as Apache will match on the host record and then determine which site to load (i.e. the domain name is the identifier). It's obviously the same for HTTPS, but then you also have to consider the host name the SSL certificate was issued to.
This is how users can set-up, upload, and test their sites prior to moving DNS.
The use of mail.domain.com, ftp.domain.com, smtp.domain.com etc in this kind of setup is more about just following convention. It's also less work if you were to say shift email off at some point to someone else but keep the website with UH. You could then just update the appropriate email host records (mail.domain.com and smtp.domain.com) and be away without having to update each individual user's settings (in theory anyway!).
Ben