Reseller accounts allow you to assign a number of user accounts to a "reseller". If you plan to sell SiteCenter as a service, you can have resellers act as your middle men, charging them on a per-user license. They can then on-sell your SiteCenter user accounts for a profit. Resellers have access to your SiteCenter control panel to add and edit their user accounts, but cannot modify settings or access any other part of your control panel.
