Before you can add any service to your server, you need to create a Customer, Subscription or Webspace that will be able to use these services.
Subscriptions and Webspaces
In Plesk Onyx Subscriptions and Webspaces are essentially the same as they both assign space that hosts either single or multiple domains. The main difference is that you can only assign Customers or Resellers to a Subscription whereas Webspaces only have their own unique user. This is because Service Provider View is aimed at hosting companies to help manage individual customers. As such a Webspace can only be assigned when using Power User View and a Subscription can only be created in Services Provider View.
Important: Each domain will need its own Webspace or Subscription if you intend to assign SSL to a domain or have separate mail or scripting settings.
The server administrator has full access to all features of the Plesk control panel. This includes the ability to create all other user accounts as well as the ability to manage server wide settings.
Reseller accounts are designed for your customers to sell your hosting services to their own clients. Resellers can create client accounts for their own customers, as well as creating hosting and email services, but they cannot create other reseller accounts.
Client accounts provide end-user access to create individual hosting and email services. Clients cannot resell any hosting services.