Plesk 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.
For users who need to resell hosting services or host numerous domains/web sites, you should create client accounts. And then, your resellers can create domain administrator accounts for their own customers. A domain administrator's account can be created only after the user's site is hosted on the server. A domain administrator account is limited to having only one 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.