Parallels Plesk Panel users hierarchy is limited to three user levels: client accounts, domain administrator accounts, and e-mail user accounts. All these types of users can have access to control panel for managing their own Web sites and e-mail accounts.
Client accounts are the types of user accounts you will create for your customers. And then, your customers can create domain administrator accounts for their own sites. A domain administrator's account can be created only after a site is hosted on the server. A domain administrator can manage only a single site; however, a domain administrator can provide other users with mail hosting services under his or her domain. For that purpose, e-mail user accounts are created and granted access to control panel.