What's the use of a domain controller and how to configure the same?

The transport domain contains all SAP systems whose transports are to be administrated jointly. One of these SAP systems is chosen as the transport domain controller.
Since all activities relevant to the entire transport domain, such as configuring transport routes or RFC connections, can only be carried out on the domain controller, the SAP system selected to be the domain controller should have:
High availability
A high level of security
A high level of maintenance

The domain controller is normally configured on a production system or a quality assurance system. Out of several reason you may choose to set up an SAP System dedicated to tasks that may be carried out CENTRALLY. This includes the task of the TMS domain controller.
The system load on the SAP system that is chosen as the domain controller is low, and only increases for a short period when the TMS configuration is changed.
Configuration:

To activate the TMS on an SAP system, run transaction STMS in client 000 with user authorization S_CTS_ADMIN.
If the SAP system is already assigned to a transport domain, the TMS initial screen shows the system’s status, either as a domain controller, or as a SAP system within a domain.
If the SAP system is not yet assigned to a transport domain, the system tries to locate an existing transport domain within the current transport group by reading the file DOMAIN.CFG in the directory bin of the common transport directory. If no transport domain exists, a new transport domain is created, and the current SAP system is assigned as the transport domain controller.
When a transport domain controller is first created, the TMS generates:
ü A transport domain and a transport group
ü User TMSADM
ü The RFC destinations required for the TMS
ü File DOMAIN.CFG in directory bin of the common transport directory, for storing the TMS configuration
o The name of the transport domain may not contain blanks and may not be changed afterwards without reconfiguring the domain controller. By default, a transport domain will have the name DOMAIN_, where is the system ID of the domain controller. The name of the transport domain cannot be changed afterwards.