When managing multiple domains for multiple people or businesses administration can become complex. Often users require a login to manage their domain name themselves, whether it's to make a payment or to change the contact details on a domain.
Sub-users are user accounts that have their own login identity and password but are associated with your account. You have great flexibility over which resources and domains a sub-user can access.
You can even assign certain users as `billing` contacts - these users can manage invoice and payments but do not have access any other domains or resources.