The master realm in Keycloak is a way for Keycloak admins to create and manage all of the realms on their system. In order to manage them, HCI creates the Keycloak admin super user during installation. Where HCI admins have full access to the HCI realm, this super user account is assigned unique privileges for access to all realms on a deployment.
While additional HCI admin users can be created on a system, only one Keycloak admin can exist.
The master realm can be accessed in Keycloak by using the Realm Selector dropdown from the left-hand menu and selecting master.