A global administrator is created when replication is enabled, and is responsible for managing configuration server groups. A configuration server group is an administration domain that allows you to synchronize configuration changes to one or all of the servers in the group. For example, you can set up a group when configuring a replication topology, where configuration changes to one server can be applied to all of the servers at a time.
Global Administrator(s) are stored in the topology registry. These entries are always mirrored between servers in a topology. Global Administrators can be assigned privileges like other admin users but are typically used to manage the data under cn=topology,cn=config and cn=config. You can create new global administrators and remove existing global administrators using the dsconfig tool. The global administrator entries are located in the cn=Topology Admin User, cn=topology,cn=config branch.