How can define primary domain controller or additional domain controller?
On Windows Server Systems, a domain controller (DC) is a server
that responds to security authentication requests (logging in,
checking permissions, etc.) within the Windows Server domain. A
domain is a concept introduced in Windows NT whereby a user may be
granted access to a number of computer resources with the use of a
single username and password combination.
Additional Domain Controller (ADC) or Backup Domain Controller
(BDC) is a backup computer hosting DC services to be used as a
backup in case your primary DC goes down or is somehow
unavailable.
The Primary Domain Controller (PDC) and BDC synchronise their
data on a regular basis so that either can be used as DC.