The Unix Username is the username of the owner of the domain. This user will be permitted to edit the htaccess options for the domain website, as well as edit the website content. This user may be used to receive email, though it is traditionally named after the domain, and is not useful except as an optional catch-all email account (catch-all email addresses are not recommended anymore due to the spam scourge...perhaps some day, when spammers are imprisoned for their actions we can have catch-all addresses again).