A Managed Account provides the customer with access to their SIM estate. This access allows customers to monitor, manage and report on their SIM estate. Associated with each Managed Account is at least one user of type Admin, and zero or more users of type User and View Only. See What are User Roles? for more information on what users can do.
If a customer re-sells SIMs to a sub-customer, an Admin user can create a sub-account for that sub-customer which will allow that sub-customer to manage their own SIM estates. Sub-customers can also sell SIMs to their sub-customers, and so on.