The system predefines one root domain that is named Root by default and has special characteristics: The root domain cannot be deleted and each user has the permission to list subordinate domains of the the root domain. However one can rename the root domain, change permissions and set other properties of the root domain if either the system permission Domains: modify root is granted or if privileges are granted  explicitly on the root domain

The root domain is the only domain that has no parent domain, in other words all user defined domains have a parent domain.